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-   -   Hafler DH-200/220 Mods (http://www.diyaudio.com/forums/solid-state/31131-hafler-dh-200-220-mods.html)

fab 28th March 2004 03:49 PM

Hafler DH-200/220 Mods
 
2 Attachment(s)
As suggested in "hafler dh-220" thread to repair it, here is the starting point of mods that peoples have done on this amplifier.

So far in previous abovementioned thread, 2 mods were brought up:
#1) Replacing the load resistors in input stage with current mirror. I believe that this mod would increase the open-loop gain of the amp, thus would increase the amount of feedback. Is it good or bad? or can we re-adjust the gain of the input stage or play with the gain of the driver stage so the overall open loop gain is not modified?
#2) Regulating the power supply of the circuit (except the mosfet output stage!). This one I have personnaly done. See attached principle. The regulated voltage can be higher than indicated on the schematic so the maximum power loss is not much. Has someone else tried this mod #2 also? Even my girlfriend (who are not an audiophile) noticed the improvement on the sound (she was not trying to be kind with me...).

Variac 28th March 2004 03:56 PM

This thread is long overdue. There are a LOT of HAfler amps out here....

djk 28th March 2004 04:35 PM

"Replacing the load resistors in input stage with current mirror. "

Who ever suggested that has never seen the schematic, won't work. The VAS devices are biased by the DC developed across the load resistors, its a complementary design.

"Regulating the power supply of the circuit (except the mosfet output stage!). "

Great idea. The ideal would be 7V~10V above the nominal 60V rails. A non-feedback regulator sounds the best, a pass transistor with a zener in the base.

Replace the main filter caps with 15,000F~20,000F and add a pair of 47F bypass caps. Replace power switch and add 2 ohm inrush current limiter(thermistor). Replace the 16ga ground buss with a 3/8" copper strap.

Add a pair of Schottky diodes, one in series with the bridge + and - outputs going to the main filters.

Replace the 0.1F cap in the output zobel with a 250V foil and film type.

Once the regulated supply for the front end is in place, remove the CCS for the diff inputs and replace with a pair of resistors.

Cascode the diff pairs and return the resistor tails to the opposite reference voltages(see the Leach V4.5).

fab 28th March 2004 06:20 PM

Yes, it is long overdue (you can check on Ebay to see the amount of dh-200/220 amps that are sold!)

#1)
""Replacing the load resistors in input stage with current mirror. "
Who ever suggested that has never seen the schematic, won't work. The VAS devices are biased by the DC developed across the load resistors, its a complementary design. ""

fab:I am not the one who suggested it but nothing is impossible. In Elect. & Wireless World issue of January 1985, it was described a symetric complementary discrete op-amp with current mirrors for negative and positive loads of input pairs. Question: Using the referenced Leach v 4.5, could Q8 with R21 imposes a fixed DC voltage at base of Q12 so a current mirror could be used (a course the same would apply for the negative complementary part of circuit)?

#2)
""Regulating the power supply of the circuit (except the mosfet output stage!). "
Great idea. The ideal would be 7V~10V above the nominal 60V rails. A non-feedback regulator sounds the best, a pass transistor with a zener in the base."

fab: Where do you get the additional 7 to 10 v (use of another transformer?, voltage doubler?) ?

#3)
"Replace the main filter caps with 15,000F~20,000F and add a pair of 47F bypass caps."

fab: I used 2 x 30000uF in one amp and 4 x 14000uF in another. For the latter, I used 2 x 14000uF with one bridge for the positive supply and used 2 x 14000uF with one bridge for the negative supply. The 47uf along with 100nf bypass caps were installed directly between the mosfet drains and chassis dissipator.

#4)
"Replace power switch and add 2 ohm inrush current limiter(thermistor). Replace the 16ga ground buss with a 3/8" copper strap."

fab: of course for the power switch and the round buss. For the current limiter, have you experienced problem in the past without it or is it precaution measures?

#5)
"Add a pair of Schottky diodes, one in series with the bridge + and - outputs going to the main filters. "

fab: what is the advantage?

#6)
"Replace the 0.1F cap in the output zobel with a 250V foil and film type."

fab: is it making a big difference?

#7)
"Once the regulated supply for the front end is in place, remove the CCS for the diff inputs and replace with a pair of resistors."

fab: I prefer keep the CCS to reject more common mode signal.

#8)
"Cascode the diff pairs and return the resistor tails to the opposite reference voltages(see the Leach V4.5)."

fab: cascode of course! for the resistors, see above comment on #7.

I have some more on my side....but let start with comments on the above mentioned so far. Doing a change is someting but appreciating it is something even more important. After all, the goal is to reduce the degradation of the power amp and make it more reliable.

djk 28th March 2004 09:11 PM

"fab:I am not the one who suggested it but nothing is impossible. In Elect. & Wireless World issue of January 1985, it was described a symetric complementary discrete op-amp with current mirrors for negative and positive loads of input pairs. Question: Using the referenced Leach v 4.5, could Q8 with R21 imposes a fixed DC voltage at base of Q12 so a current mirror could be used (a course the same would apply for the negative complementary part of circuit)?"

Q8 is part of the short circuit protection and being engaged all the time would present such a heavy load as to negate the advantages of CCS loading. I don't like the sound either.

"Where do you get the additional 7 to 10 v (use of another transformer?, voltage doubler?) "

Easiest way to get it is to use a dual 5V transformer and add one winding to each end of the main transformer.

"I used 2 x 30000uF in one amp and 4 x 14000uF in another. For the latter, I used 2 x 14000uF with one bridge for the positive supply and used 2 x 14000uF with one bridge for the negative supply. The 47uf along with 100nf bypass caps were installed directly between the mosfet drains and chassis dissipator."

30,000F is right at the edge of bridge failure. I also find with a regulated front end that it reduces the need for massive filters for the output stage. For 60hz the point of diminishing returns for a single 8 ohm load seems to be 3,300F or 6,800F for a stereo amplifier, and double that for 4 ohms. For 50hz I go with 4,000F. The bypss caps should be attached to where the speaker pulls power from. For the V that would be at the outputs, but the ground is at the transformer center tap. I find it works best to put the 47F right on the main filters and the 0.1F~1F at the outputs. The smaller value can break into oscillation due to stray inductance from the wiring. Sometimes a small resistor in series helps, sometimes I have to remove the smaller value.

"For the current limiter, have you experienced problem in the past without it or is it precaution measures?"

Power switch failures, bridge failures (above 30,000F), thermostat failures (mainly DH500 and Adcom GFA555).

"Schottky diodes, one in series with the bridge + and - outputs going to the main filters. "

fab: what is the advantage?"

Reduce rectifier noise. I find it much easier than trying to replace the whole bridge and make a heatsink for same. On any Crown bridge design (Power Base, Micro Tech, Macro Tech, etc) you only need one diode.

"Replace the 0.1F cap in the output zobel with a 250V foil and film type."

fab: is it making a big difference?"

Think about it. 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. A 250V foil and film type with soldered leads is best. Most film types have paste and mash lead termination, not so hot. The resistor in series must not burn up.

"fab: cascode of course! for the resistors, see above comment on #7."

Leach agrees with me, CCS sound bad (even though they measure better).

Actually, the best thing to do with a Hafler is lose the FETs and convert it to a Leach.

slowhands 29th March 2004 12:18 AM

Quote:

Originally posted by djk
"Replacing the load resistors in input stage with current mirror. "

Who ever suggested that has never seen the schematic, won't work. The VAS devices are biased by the DC developed across the load resistors, its a complementary design.

......

Actually, yes, current mirrors can safely replace the load resistors in this amp. The technique is commonplace in IC design, I certainly did not invent it. The mirror is a differential Darlington structure, which acts as both an excellent mirror and a buffer to the next stage and has well defined output levels to bias the VAS.

As for the other changes suggested, feel free. I don't see a return on investment for most of them, however. The sonic improvement for the current mirror loads is high and it is not difficult or expensive, so I place it at the top of my list. Regulating the front end supply is expensive, and the payback is possibly not so dramatic, but again, feel free...

If you want a Leach amp, build one, don't modify the Hafler to be one. Certainly any design that has lasted as long as the Leach amp must be pretty good. I'm impressed that it is on revision 4.4, and that it has been critiqued by everyone and continuously improved since 1977. The Hafler 200 was designed in 1979, and also continuously improved in later products in the Hafler product line, so they must be doing something right there too.

slowhands 29th March 2004 12:43 AM

Re: Hafler DH-200/220 Mods
 
Quote:

Originally posted by fab
.....#1) Replacing the load resistors in input stage with current mirror. I believe that this mod would increase the open-loop gain of the amp, thus would increase the amount of feedback. Is it good or bad? or can we re-adjust the gain of the input stage or play with the gain of the driver stage so the overall open loop gain is not modified?

It will increase the open loop gain, about a doubling. So either the compensation should be re-analyzed at the same time, or the input pair degenerated further by increasing the emitter resistors to return to the same gain and stability. I have not done this mod to this amp, so I will not recommend either choice. I have done the current mirror mod to the Citation MF12, with very excellent results.

I want to do a new driver board for this amp that incorporates the most effective improvements and uses modern available plastic packaged lateral MOSFETs (and possibly vertical MOSFETs, with appropriate biasing). It has become expensive to replace the old TO-3 packaged Hitachi's, and I think all the hand wiring to the outputs can be eliminated too. So I will be breadboarding this on an existing amp soon, hopefully prior to going to new PC boards. Stay tuned.

fab 29th March 2004 01:03 AM

2 Attachment(s)
thanks for the reply. Your comments are greatly appreciated.

Maybe this thread is going a little bit too technical, isn't it? It would be fun to get also other peoples (Variac, Larryg where are you?) involved as well.

For the speaker ground, I have connected it directly at main filter caps instead of passing through the pc board as suggested by Musical-concept (see attached file because it looks like their site has been hacked!)

djk said:"Actually, the best thing to do with a Hafler is lose the FETs and convert it to a Leach."

It looks like you have a big preference for the Leach amp! I respect that but the goal of this thread is to improve the actual DH amp with easy low cost mods. I like too much the "mosfet" sound (maybe because I learned to work with them from the beginning). Mr Pass, Borbely, Halcro seem to agree with me on that! I may repeat myself but all the mods I have done made quite a difference over the stock DH-200 amp sound! I believe that with a stock DH-200/220 and a few bucks for some mods can make this amplifier sounds like one at several thousand bucks...My speakers are not hi-fi but only "mid"- fi (Boston Acoustics VR-30).

Getting back to one specific mod (regulated front end power supply), I used a 78xx series regulator with Zener diode to increase the voltage output. The ripple obtained was extremely low, which preserves the stability of operating points of the pc board transistors. I have built this regulator circuit at very low cost on a Radio-Shack IC pc-board 276-150 (good quality proto board). With all the components, for 2 channels, the cost is less than 15$(can or us) and it fits easily inside the DH-200/220 amp. If someone is interested, I can provide the circuit schematics and layout.

fab 29th March 2004 01:12 AM

Re: Re: Hafler DH-200/220 Mods
 
Quote:

Originally posted by slowhands


Actually, yes, current mirrors can safely replace the load resistors in this amp. The technique is commonplace in IC design, I certainly did not invent it. The mirror is a differential Darlington structure, which acts as both an excellent mirror and a buffer to the next stage and has well defined output levels to bias the VAS.

I want to do a new driver board for this amp that incorporates the most effective improvements and uses modern available plastic packaged lateral MOSFETs (and possibly vertical MOSFETs, with appropriate biasing). It has become expensive to replace the old TO-3 packaged Hitachi's, and I think all the hand wiring to the outputs can be eliminated too. So I will be breadboarding this on an existing amp soon, hopefully prior to going to new PC boards. Stay tuned.

Can you draw your current mirror for our knowledge (remember that the VAS does not have a current source)?

I am workinfg also on a new driver board for the DH-200/220 (I have one ready layout with traxmaker but I am always looking for possible improvements). Maybe we can share some hints and may end with a SUPER driver board? Imagine the power of design of many music addicted peoples all together!!

mcp 29th March 2004 04:33 AM

Quote:

Originally posted by fab

#1) Replacing the load resistors in input stage with current mirror. I believe that this mod would increase the open-loop gain of the amp, thus would increase the amount of feedback. Is it good or bad? or can we re-adjust the gain of the input stage or play with the gain of the driver stage so the overall open loop gain is not modified?
It is good, provided it can be implemented.

In a discrete symmetrical topology, where the VAS consists of Darlingtons (like the DH-220), it is extraordinarily difficult to use active loads due to the lack of precision on the input stage.

There are quite a few areas where the DH-200 can be improved upon. The driver stage, for one, is worth examining. Since Power Mosfets can be driven directly from the VAS, the use of drivers in the DH-220 implies that the VAS could be biased slightly on the lean side. With these drivers removed and the VAS re-biased to about 15mA, I would imagine the character of the DH-200 to change significantly.


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