Restoration of the HAFLER DH200 power amp

[SonicArt]

For such a popular kit amp, thousands sold, many of them here in OZ, there is little talk about Haflers in SNA, a search showed up almost nothing bar some for sale items from years ago.

 

I happened across this DH200 and was surprised no one had jumped on it. Might have been the fact it looked rather ratty with a bad respray and was running on a stepdown transformer (which many of us dont want to deal with, me included) but I wasn't going to argue with fate! It was running when I arrived to check it out, temp seemed balanced on the heatsinks, and looking inside it had a lot more primary transformer windings than what you would expect a domestically sold Hafler to have!

(Fill in info, Haflers came with either a US transformer, 120vac windings, or an International transformer, which could be configured to any world voltage and had 6 winding taps).

So I get it home, pull the cover and stare at the wiring. (having printed out all the paperwork, diagrams etc already). I reckon I stared at it for 30 mins trying to get my head around why it was running on a 120v stepdown (which he wasnt selling) when it appeared to have the International transformer in it,

Measured the switch poles, studied the drawings, the wiring colours matched up perfectly to a 240v configuration, so why was it running on 120v  and suddenly things clicked! The switch was wired backwards!  

Ok so switches on Haflers, especially DH200/220/500 dont last, they dont have inrush current limiting so the switch gets hit when you turn the amp on and eventually wears out. This one was replaced. Now the original switch is centre pin  AC in, pin 1 AC out to the thermal breakers and the open circuit lamp, (and also the internal switch light), earth pin 3 is part of the internal switch light circuit, it goes via the jumper to the Primary windings that are connected together to series the windings into 240v (being 120vac winding point).

So if you dont measure your new switch to check the connections and just put it in the same as the old one, you end up feeding the 240v to the primary windings series connection point! No wonder it was running on 120V!

 

All that was needed was rewiring the switch the correct way, and bingo, 240v hafler! Of course I powered it up on the variac to be sure! At 240v the main pwr supply was sitting at 58v per cap, 116vdc total, pretty close to expected.

Once it was running on 240v I hooked up all the meters, DC offset was around 50mv in the left channel, and 10mv in the right, Bias was pretty close to factory, so I set this to spec once it was warmed up.

 

DC offset in the DH200 is not adjustable, they achieve a low offset with matched pairs of differential tansistors. A good way to do it I guess as it removes the need for a trimpot and circuit to adjust dc offset which if I understand correctly introduces distortion/noise? into the circuit.

More on this in future postings...

 

Next up check all fuses, clean terminals, ready for a listening session to get a base point before stripping the old girl down. Speaker output fuses were 5A instead of 2A, and one spring insert to hold the fuses in the cap was long gone, some industrious person had replaced it with some stainless steel wool!

I "borrowed" an insert from another new fuseholder, and cleaned all the contacts, got it all together and hooked it up.

Running the Sonic Frontiers into it the little Dh200 sang, it sounded  really good! It was a little soft, bass a little loose, but it is a 1980 model amp with all original electrolytic caps, considering the life its obviously had, it was still showing what it was capable of.

 

Next up stripping the old girl down, comparing the two channels and making a list of things needed!

[GregWormald]

I have a DH-101/200 combo that I just can't bear to part with (so far). It was, and is, a nice piece of gear considering its era.

 

I had it made up from the kit by a good tech, as it was cheaper than the factory  version. A friend did the fairly standard upgrades to the 200 which included a higher bias, some capacitor changes and IIRC some other stuff. (I think I still have all the original books plus the upgrade articles and plans.) This was years later and the sound improved a fair bit.

 

Trevor (Rage Audio) serviced the 101 not too long ago and I used it and the 200 with some old AR speakers while my new actives were being manufactured.

 

When the new gear was all here and set up, I was quite prepared to give that little set away to some friends but couldn't find one that was interested—music just isn't part of my friends' lives like it is mine. I think I'd have a hard time surviving without music.

 

Let us know how you go.

[SonicArt]

Ok so with the 240v sorted and prelim listen out of the way, it was time to pull the old girl apart and see what makes her tick....

 

Here are the insides front and back, all original as it came:

 

Its amazing how little hum you get from a Hafler with all the spaghetti wiring and that twisted pair for the inputs. The terminals are very average nickel posts and the RCA inputs have seen better days, the fuse holders are going to have to be tossed as well due to the missing parts and the condition of their contacts. Cant be too hard on it though, it was a kit amp and built in 1980!

 

Checked the main filter caps just to see how they were doing, not too bad although the Vloss is pretty high, being 75v caps and running at 60v in circuit you dont need to lose much rating to be a concern. The capacity is still impressive though, considering they are 10mF caps and 43yrs old! Still they have seen their day in this amp,

 

 

I have some NOS Cardas connectors sitting in a box I have kept for my own projects, back when they were really affordable! Binding posts for example back when the AUD was almost 1:1 cost me about 25 bucks per assembly (channel) for pure copper silver rhodium plated models, (see them in pic below with the power caps removed and the back starting to be disassembled) Now the cost is frightening!

 

Next step remove the amplifier channels, 4 screws hold each heatsink on, unscrew them, move them out and take photos of all the wiring! You need to unscrew the boards from the heatsinks in order to access the backs to carefully unsolder all the connections.

 

Looking down the back of the board at the gate resistors and the drain to ground cap, well it doesnt appear to be connected to the drain, its connected to the gate with the resistor...hmmmm. At least the gate to source cap is where it should be.

 

Both channels are now out on the bench, comparing the two there are a few components dissimilar, replaced over the years when they have gone out of spec or failed I guess.

The two main electrolytics on the boards are 100uF 80v and are the power supply decoupling caps for the input and VAS. Research has suggested that 220uF is the magic number here, (thanks to all the Americans modding Haflers over the years and testing various components makes life a little easier!) I have Panasonic 220uF 100v FC on order for this spot.

The light blue cap is a 470uF Bipolar, in the feedback loop, I have Nichicon 470uF on hand, I dont have any MUSE series (the green ones you see a lot in mod threads) but I will order some when the opportunity arises, for now the regular BP will do.

 

Below is a closeup of one board, the blue can at the top is the input cap, its a 10uF Bipolar, I will replace this with a 4.7uF film cap. it feeds directly into the 2.2k resistor next to it, I will be putting original NOS Beyschlag in there (and many other spots) as I have a box of them from when I used to rebuild Leak and Quad amps amongst other tube stuff.

 

Thought I would post up the AC input side of things too while access is easy.

AC power in is a fixed lamp cord Well maybe not that bad but its pretty basic. The wires are lined up as per the switch connections. The original wiring sees the Neutral entering as the centre wire in the pic, lower wires are white to Thermal breakers then primary winding, black to the open circuit lamp, the green is the jumper going to the centre of the primary winding (120v) to complete the switch lamp circuit.

 

Cutting out the rear panel with a nibbler and fitting an IEC socket (a furutech one as I have them on hand and they are not very expensive) was a must as all my amps run IEC and being able to swap them over easily is important.

Rewiring with some cores from a piece of scrap Evolution power cable gave me colour coding, this time the neutral was run to the primary winding and the active through the fuse then a thermistor to control inrush current (yes I know old school, eventually I will sort out a soft start circuit) then to the new switch with a new X cap across it.

 

Next up the chassis, and that horrible paint job!

[SonicArt]

For the case top, I decided to paint strip the whole thing, as it was rough as guts and I figured there was going to be some corrosion spotting underneath as well as all the scratches and marks.

Unfortunately I cannot find my photos of the top during the refinish, very annoyed as the stages looked really good in pics...

Anyway, it stripped very fast, within minutes I was scraping off all the old paint, then I used a fibre wheel in the drill to rip off any hard to get at spots and clean up the steel surface. It was a bit spotty with some corrosion but very light and on the surface, most of it came off.

The heatsinks were also covered on the front/back faces and tops, I stripped them off and cleaned them as best I could, they have a lot of the anodizing rubbed off on the fin edges, but its either strip them and have them re-anodized ($$) or leave them as they are, they look ok, and this is supposed to be a budget conscious build (to some extent )anyway...

 

I then used primer filler on it, acrylic car paint basically, having experience with acrylics over the decades (painted a few cars when young!) its easy to use and you can get a nice finish from a spray can.

3 coats then rubbed back with 800, another coat and light rub with 1200, then I used Septone (as its the only easy to buy off the shelf satin black close by) acrylic which turns out to be really good paint, it goes over the SCA primer nicely. I got it too smooth for the first 2 coats, so final 2 coats I sprayed it a little lighter and got a little peel effect in the surface, looks better on the vintage chassis and its more satin looking this way.

 

The finish came out great!  I wont show you yet, wait till the amp is finished and back together...

 

The lower chassis had been painted across the front, the back was left alone due to all the screen printing on it.

I found a couple of pics of this, so here is the front being primed:

 

And the front in its satin black paint:

 

[SonicArt]

Quick pic of the case top, looks a lot better than it did in the first post of this thread!

[mbz]

Consider fitting some speaker protection. I wouldn't be relying on spkr fuses, too slow.

[SonicArt]

Its on the cards, couple of options out there I am looking into that have been used in the Haflers.

[SonicArt]

Ok better get back into this little project!

 

So I had the heatsinks with their boards removed from the chassis, the chassis all masked up being painted.

 

While painting I set about replacing all the mismatched components, and a list of parts that are important to the balance of the circuit, the differential transistor pairs, and a bunch of resistors in the circuit that need to be close to each other in value.

 

I often use Vishay 1837 & Wima FKP for replacing old polyester etc film caps. The Hafler has a few film caps on the board and some mica as well. I wanted to keep it reasonably close to the original design, albeit with much better quality parts, so the Mica were replaced with new US made silver mica caps, all the film were replaced with 1837 and FKP2.

The input cap is a 10uf bipolar can, I replaced with an Axon 4.7uf true cap, chosen from all the feedback and notes I have collected from the myriad of US mod threads in other forums.

Resistors, I used my stock of Beyschlag where possible, primarily around the input stage where drifting resistors can cause excessive DC offset, and increase distortion due to imbalance in the differential pairs. I had 100pc bags of these so was able to measure and match up sets of resistors for each position.

Where I didnt have the value I ordered some 0.6w metal film (stock are 1/4w unless specified in the parts list).

 

Picture below shows original channel top, 1st stage rebuilt one underneath.

All 2.2k resistors were replaced with 1w Beyschlag, they are good for 2w in reality, a little big for the board but I had them and they are excellent sounding resistors.

The main filter caps were 100uF 80v, replaced with Panansonic FC 220uF 100v, this size was chosen after researching what others had tested, it was the recommended as the best balance.

The 0.1uf cap in the zobel network was replaced with a hi voltage solen poly, the reason for this was explained by one of the US guys in a Hafler mod thread:

The charge absorbed by the dielectric has to discharge when the waveform crosses zero in quiet passages. With no signal to mask it this discharge sounds like noise and hash. The currents through here can be huge. A 250V film cap may only be rated at 10V above 10Khz.

 

After doing the first stage on the boards, replacing the rest of the important resistors when they arrived, I replaced all the original wiring with 16AWG PTFE silver copper alloy hookup wire, every bit of it, I figured while the 2 channels are off the chassis what better time to do it!

 

The main 10,000uf 75v filter caps were replaced with Kemet 20,000uf 100V caps, same size can and screw terminals so they dropped into place with no mods. I also cut a ground bar to join the caps from some 1.6mm thick brass strip. I would have used copper but the local model shop didnt have copper strip and I didnt want to drive far afield just for that. The brass has worked out nicely, will get some good pics of all the power supply mods later.

While rewiring I replaced the old 25A bridge with a new onsemi 35A bridge. new cap across the bridge to replace the old disc cap.

 

Once this was all done I refitted the channels back to the chassis with the new wiring, and added a shielded PTFE silver copper twin core wire for the input.

 

I also changed the grounding on the input connections, the original amp has one side floating and one side grounded to chassis through a 2.2ohm resistor. I decided to follow what others had done and drill a hole behind the bridge at the rear centre of the chassis, bolt some solder tags to the hole, and then run ground wires from each new RCA jack to the new centre ground tab via an 8ohm resistor. Some guys used a pair of 2,2ohm, some used a single 10 ohm, I had 8 so I used it!

 

Speaking of RCA's, here is a quick pic of the back of the amp, maybe a little overkill if you had to buy all these fittings today! yeah ok I would NEVER use Cardas if I had to buy them now, the speaker posts cost more than the amp! But you have to admit it does look a little sexy...

 

After it was all together I set up meters for checking dc offset and bias. I set the bias to 290mv, a little more than factory DH200, little less than a DH220 (which runs the same devices).

 

DC offset was sitting nicely in the right channel at 5-6mv, the left channel was a little high at 43mv.

 

So I sat down with my little transistor tester, and a bunch of 2N5401/2N5551 that I had ordered a few weeks ago. These were supposedly 2002 NOS Fairchild transistors. I managed to get 5 lots of matched devices from the 5401, and the 5551 measured into 2 lots only which was impressive, only a couple of odd devices.

 

I colour coded them with red and white paint, and then replaced Q1-6 in the left channel. Q1&2 and Q5&6 are the differential pairs, these need to be closely matched in order to minimise the DC offset.

 

Amp back together and wow, the little $30 transistor tester is not a bad little jigger!

DC down to 5.5+/- on the left, gotta be happy with that...

 

I will stop here for now, there is more to this little amp than where we are at, suffice to say it is sounding quite nice, I put some hours on it to break in all the new parts, and a few little things have shown up during some parts upgrades, will get to that in the coming posts along with some finished photos.

Edited July 29, 2023 by SonicArt
added pic

[SonicArt]

Continuing the saga of the DH200 rebuild....

 

The feedback circuit uses 470uF Bipolar caps, all I had was standard Nichicon BP, Looking around I wasnt going to be able to buy some green MUSE series without waiting for them.

I thought have a look through the stash of BG, low and behold a pair of 470uF 16v red bipolar Black gates!

 

Why not, so I set about fitting them, left channel went nicely, all looking ok, right channel I had removed some of the little heatsinks off the metal can transistors, putting them back on an lining them up must have upset one of the old 2N5415's, of course I wasnt aware of this at the time it all happened, but after re-assembly I get 250mv DC at the outputs!

 

This of course had me going back over the whole channel, checking and rechecking devices and components, drove me mad for days, all the while my gut telling me something is not right with that metal can trannie. I pulled it and measured it, the gain was low, but it measured as a transistor within parameters. I swapped out the VAS devices, differential pairs, nothing was glaringly obvious, offset was still at 220mv odd.

(Side note, I had been monitoring bias and noticed it had dropped down, so adjusted that back up while troubleshooting the channel)...

 

Start of the week I decided to order some 2N4515 from a local supplier, ended up with ST micro which are nicely made devices.

When they arrived I measured the 4 and they were all close, and significantly higher in gain vs the suspect device. I duly popped a new one in and my DC dropped from 221mv (at this stage) to 20mv! Looks like gut was right this time

Bias also jumped from 290ma to 450ma, reiterates the suspect device was weak.

 

Reset bias, and then looked towards getting that 20mv down.

 

Revisited the differential pairs, pulled one pair at a time and re-measured, wasnt 100% happy so checked more of the stash and matched another pair, popped them back in and DC down to around 11mv! Next pair was checked and optimised, DC dropped to a couple of mv! Gotta be happy with that.

 

So disaster averted and the amp was ready for some more running in again. Maybe that transistor was on the way out and disturbing it pushed it closer, not being a trannie whisperer I will leave it at that...

 

Photo showing bias on the outside with the two micron meters showing DC offset. Am pretty happy with how things have ended up, and I am going to buy a couple more of the micron meters as they are really good little meters, appear to be quite accurate, and when I tried them out on the bias I was able to read to 0.1ma as they have a low range of 0-600ma which is much better than my old meters 0-200ma.

 

 

Edited July 29, 2023 by SonicArt

[Hafler Fan]

Hi, I have been following you work for a few days and it is the same as I have done on my DH200. My issue is that the driver transistors seem to run quite hot. My bias is set to 250 and dc offset is 3 to 5mv. The mosfet are warm as normal. Are your 2N3440 and 2N5414 also hot?

 

[SonicArt]

I don't recall the drivers being hot,  warm only but I would need to pull the cover and check to be sure.  might be able to do that later in the week.

[mbz]

Maybe consider a triac to take the heat off the power switch.

https://audiokarma.org/forums/index.php?threads/is-your-unobtainium-power-switch-worth-5-and-an-hour-or-so-of-your-time.504673/

[Hafler Fan]

I will measure the temperature tonight. A proper reading may be more usefull. The amp measures good, bias/dc offset, and sounds great. My concern is the heat will cause premature failure. 

[Hafler Fan]

From your post I am assuming our amps are very similar.  I have not got to the paint stage yet. 

[John N]

Under Rated the Haflers. I have a DH 220 that I purchased  from Penny Lane new it has and continues to be a good performer to this day. Only ever had to replace the on/off switch on 2 occasions since the early 80's.

Recently had it upgraded and refurbed by Mark ( Just because) and it was money well spent and sounds better than new.

I suspect this Amp surprised Mark who now is a Hafler owner ( and many others who are

not wise to them).

Mine will always be in a system and enjoyed.

Go Mark.

[Hafler Fan]

They are great performers even by todays standards. I had to replace 3 transistors after 39 years of operation...not bad. It is now refurbished and sounds great, but because I almost blew an expesive set of speakers I am going to add a protection circuit. 

[Hafler Fan]

The speaker protection is installed and working perfectly. A loose/damaged ground wire was the cause of my problem with the speakers. Probably my own fault. At 25C ambient my heatsinks are 35C, but the drivers are 55C. I rechecked all the components so I guess the driver transistors do run a  little hot. Bias is at 260mA after about 30min, when cold the bias is 275mA and offset is 5 to 8 vdc. I implemented some of the refinements of the dh220 and also rewired each channel to star ground. I am very happy with the sound of this amp with my Canton GLE 90's. Next my dh101 will get a few uprades and a refurb. The selector switches are really temperamental and a remote volume control is a must these days. I use the dh101 to switch on the amp. This saves the switch on the amp. Mark's post was really helpfull and well written. Great work, thanks Mark.

[GregWormald]

I had Trevor at Rage Audio renew my 101 (with a few unresponsive selectors) and it came back just like new.

[Hafler Fan]

I bought 2 relay boards, 8 relays in total, and my idea is to rewire the unit using the switches to activate the relays. it is still a work in progress. I will upload some photos when I have figured it out. The remote volume control works well a so do the new bass and treble controls. The rear RCA connectors have also been replaced so only the switching is left to do. The service industry here is ''not good'' so I rather do these jobs myself...and with a lot of google research.

[SonicArt]

Been a while since I spoke about the Hafler! Been so busy this year with projects and work, but I did find time to refine the old Hafler power inrush issues and the speaker outputs...

 

So first up was the power inrush, I found a nice soft start board from a Chinese supplier, really well made and not expensive, it was going to be a tight fit but I was determined to sort out the AC supply permanently. The board is simple and neat, it uses large parallel thermistors on the active side, they are bypassed by a large 100A relay. Built in circuit operates the relay with a 3 second delay on startup. Makes life easy, AC in, AC out, nothing else needed.

I have few photos of mine as I was nutting it out as I went along and forgot to take pics!   but I have since done the DH220 I worked on so most of the photos are from it, identical setup and mounting though.

 

I machined up standoffs after working out where the board could sit, not a lot of space in the small Hafler chassis! The end result has the board sitting upright on vertical posts, milled flat at the top and tapped for screws, it worked out really neatly.

 

The board uses one of the factory tag strip holes (same in both amps), all you need to do is remove the bolt and run a 4mm drill through the hole, the 4mm button head screw holds the rear post and the tag strip in place. The front post needs a new hole in the base of the case, simple job to measure the position and drill that. End result as seen below:

 

I left the tag strip in place as it holds the paralleled windings (and the circuit for the fault light in the DH200). The power comes in as usual from the IEC socket, active into the fuse, from fuse to switch, from switch to the new board, out the new board to the tag strip point. Neutral goes straight to the board, out the board to its original TX winding point.

End result is the board works great! No thump feeling or arcing through the switch, its an anticlimax to switch it on now, measuring the power circuit and you can see the slight delay in charging the main caps, just enough to remove the dead short thump they usually receive from old 80's Hafler setups.

 

Sound test and no change, I let it all settle in for a few hours then evaluated the amp, dead quiet on the output with no input connected, very happy with the end result, time to move on to the speaker protection!

[John N]

Mark fitted the soft start and speaker protection to my DH220 and along with his previous upgrades and refurbishing. I feel more comfortable with using it even though in the 40 + years I owned it I only ever blew one speaker fuse and

replaced the on/off switch on 2 occasions.

And as is Mark way his workmanship  is superb.

I will probably give him my DH 100 preamplifier to have a play with as it is untouched from new and I'm certain he will find some worthwhile improvements to implement.

[SonicArt]

So speaker protection was always at the top of my list of things to add to the Hafler, the original design sees a wire coming from the board, out to a fuse holder with a 2A fuse in it, and then back to the board. The fuse holder has a small pf polystyrene cap across it so if the fuse blows all you hear are faint high frequencies...

The speaker outputs then come back out from the board to the binding posts, earthed to the main circuit earth.

Agricultural at best, but it was the late 70's early 80's and it was a budget kit amp

 

I know there are a few speaker protection boards around, the Chinese make some that look reasonable, there is a guy in the US making a kit for Haflers, long black box with wires you connect up to the boards and the posts, box sits in the back of the chassis. His board runs off half the AC wave from the power transformer,  44-45v, and it splices into the existing speaker output wiring, leaving everything else in place including the fuse holders and fuses.

 

I wanted a permanent conversion, with as little extra wiring as possible, with nothing in the signal path but a high quality relay. So I sourced a soft start board from a local amp designer I have known for years, quality made in OZ board with a quality relay, all copper pathway.

I figured out the boards best position would be directly above the binding posts, so some small standoffs were knocked up and two 3mm holes drilled into the chassis just above the binding post blocks. These allowed the board to be bolted vertically with only very short linking wires going to the binding posts.

 

I then unscrewed each channel one at a time, unscrewed the board from the heatsink and unsoldered one of the loop wires that goes to the fuses, removed the fuses completely, and trimmed the other wire back and soldered it to the board creating a short link from out to in.

The board output wires were then trimmed slightly to match up with the new protection board and were soldered in place, with the earthing links attached to the board as well, keeping the original circuit path, only now it was direct from the channel output to the speaker posts via a relay.

Some blanking grommets fitted to the fuse holder holes look much neater than the horrible fuse holders!

 

All I needed to do was calculate a dropping resistor for the power input. The board was able to run ac or dc so I used half the TX output same as the US version.

A fairly big drop of 21v was needed, at 125ma current draw, gave me 170 ohms of resistance, and 2.65w of power to dissipate. I started with a 5w 180r resistor, worked fine, voltage across the relay coil was 25.3v so a little higher than I wanted. The resistor was getting quite hot with 2.6w being dissipated!

I had calculated the position of the board so that the resistor would be touching the top of the case when the amp was assembled, giving it a bit of heatsinking and original testing was uneventful with a few hours of running showing no signs of stress.

However I wanted the voltage lower on the relay, so I tried out some 390r in parallel, giving me 10w at 195r, this brought the coil voltage down to 24.3 which was perfect.

 

All in all a great result, I think the amp sounds even better with the protection board upgrade, less wiring carrying the final signal all over the place and getting rid of those horrible fuses only makes sense it would perform better!

[SonicArt]

Oh and I also covered the back of the board with wide tape, to insulate it from any mishaps seeing as the casing is rather close

[John N]

The DH220 you did for me l think sounds better than it has ever sounded.

[SonicArt]

It does sound really good John, I think the Mundorf power supply caps are one of the keys to how good your amp is, dare say even better than mine which is already excellent in the bass response.

The pre I know has quite a few little things that can be upgraded, and its supposed to sound really good when done. We could also add a remote volume if you are keen to go all the way, the volume kits are not expensive, as long as there is room for an electric ALPS pot its certainly doable!