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Greg B. 4th June 2004 07:16 AM

Hafler DH200 Mods (Spun Off From Leach Thread)
In a recent Leach Thread I wrote:

"I have a stereo Low Tim II from the late 70's and a quad mono Leach 4.5. They are great amps. Just today I picked up a clean Hafler DH200 at a local electronics swap meet. Ultimately I will do some form of Pooging on the Hafler, but after replacing bad input caps with some polypropalenes and the stock RCA jacks with some decent glod plated ones, I did a shoot out between it and 2 channels of the Leach 4.5. The Leach wins hands down. It is not subtle at all. The Leach has far more definition. Cymbals have that sheen that was completely missing in the Hafler. Massed vocals are are an assembly of individual voices in the Leach. They are a blur with the Hafler. The Leach handles all complex passages much better.

So it's on to modding the Hafler to see if it can beat the Leach 4.5. Hopefully I will be able to determine if it is the topological differences or the details of the execution that differentiate the two."

Well I performed some modifications on the Hafler and can report significant improvement. I retained the input caps and RCA jack mentioned in the original post. The additional changes I made include:

1. Replacing the diode and resistor pairs that isolate the input stage power rails from the ouput rails with 82 Ohm resistors only (ala Leach).

2. Changing the 100 uF input stage bypass caps to 100 V low ESR Panasonics bypassed by film caps and changing the 470 uF feedback cap to a low ESR Panasonic bipolar bypassed by a film cap.

3. Changing as many of the carbon composition resitors to 1% metal films as I had the correct values on hand. Unfortunately the resistors I have on hand generally have magnetic end caps so this is not consistant with true Pooge phillosophy.

4. Raising the value of the input differential pair emitter resistors to 220 Ohms to decrease the open loop gain and therefore feedback bit. This should make the input stage more linear and less prone to transient overload.

5. Rewiring the the power and ground with silver plated teflon insulated wire. The grounding scheme was changed to a true single point star ground system. Power rail fuses are no longer used.

6. Input wiring replaced with mid grade coax.

7. Speaker fuses and wiring in feedback path replaced by jumpers.

8. 2.2 Ohm resistor between input signal ground and power bypass ground replaced with 82 Ohm resistor and provided with separate wires to star point for each ground. Interestingly, when the 2.2 Ohm resistors were removed it was obvious by the scortch marks on the PCB that they had both been overheated. One was even cracked.

9. 30 Amp gold plated brass binding posts were installed for the speaker outputs.

The results were quite postive. Cymbals were much closer to the sound of the Leach 4.5. Complex passages are hadled much more clearly. It remains pretty well focused. Mass vocals sound more like an array of distinct voices. Bass was more solid. The Leach is still better in all these areas. I have to believe some of that is because it is multiple mono instead of the common supply of the Hafler.

The mods will continue and I will report the results. What mods have others made to the Hafler and what results did you get?

joe carrow 4th June 2004 07:43 AM

I've just revived a DH-500, and I'm interested to hear about these mods. It sounds like you did a LOT to that 200, very interesting indeed!

One mod that I've seen online is a dual mono conversion. I think that the Musical Concepts people have a kit (probably for the 500) to replace the transformer with two toroids.

I'm very new to amplifier modification- are there any essential mods, or things with a high "bang for the buck"?

djk 4th June 2004 10:52 AM

Have fun if you like,


Remember that it takes about 20dB more feedback with the FETs in class AB before you will get the same distortion the Leach has.

The BJT outputs have only a few 10s of milliohms impedance and that has a negative temp co. The lateral FETs have 1R7 ohms, yes almost 2 ohms on resistance over temperature and the temp co is positive.

Can you say 'mosfet mist'?

Now if you want to talk vertical MOS that runs in depletion mode, that's different. The Yamaha B2 was killer, no 'mosfet mist' here.

fab 5th June 2004 04:34 AM

Greg B.

Just to remind the peoples here that there is already a thread on DH-200 mods see
Is there a problem with my thread?

Just some comments anyway:
Most of the Leach amps discussed on this forum have a very powerfull power supply compared to the "stock" DH-200 power supply. So, to do a fair comparison, please use a smililar power supply.
Regarding the open loop gain, if you want to keep the THD lower than 0.02 at 1KHz 100W rms/8 ohms, I recommend not lowering the feedback to less than 45 db. The gain of the VAS should lowered and "stabilized" with resistors to ground so it is less depending of dynamic conditions.

Also, nobody talks about the feedback points used in the Leach where higher frequencies are not using the overall feedback at output, there must be an effect?


djk 5th June 2004 10:14 AM

"Most of the Leach amps discussed on this forum have a very powerfull power supply compared to the "stock" DH-200 power supply. So, to do a fair comparison, please use a smililar power supply."

How about the same supply?

I bought about 75 complete take-out Hafler DH200 supplies from Musical Concepts (ever wonder where your old parts go? now you know).

A stock DH200 is about 175W/4R, 100W/2R with both channels driven.

I was building mono sub amps.

Same exact supply, 300W/4R, with an extra pair of the same 10,000F cans, 500W/2R showing signs of current starvation at the limit (a 'bite' taken out of the sine wave in the second and fourth quadrant).

fab 6th June 2004 04:07 AM

"How about the same supply?"

Yes, this is what I meant, compare the 2 amps with same power supply.
The Power supply is considerably contributing in the sound of an amp.


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