Hafler DH-200/220 Mods

[larryg]

I understand that this board [19-c] is the newest board used on the dh220/dh500. Does this board have any of the improvements
being talked about?

[scott wurcer]

Originally posted by fab

#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?

After I beefed up the wiring and made other mods the in-rush current draws a huge spark across the power switch.

[fab]

Originally posted by mcp


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.

I have already done it in the past (removed the output stage emitter follower drivers and re-biased the VAS from 10 to 15 ma (changed the 100 ohms to 68 ohms in DH-200) for restore original slew rate. This had the advantage to have a more DC stable value at output mosfet gates (because of voltage multiplier compared to the voltage across the 220 ohms resistor which is varying with Vbe of drivers transistors). In fact this was done by the original designer of the DH-200 amp (Erno Borbely) in 1981 and also in Hafler 9505 for example. I confirm that it changed a lot the sound of the amplifier and was probably better at low and medium listening levels but it seemed worst to me at high levels than with the drivers. After reading some recent audio patents, and also noticing that Mr Borbely himself came back with the driver stage in his latest designs, I have been convinced that the driver stage was necessary to de-sensibilize the load seen by the VAS against the AC load at high volume level. So , I re-installed the driver stage and even installed a darlington as the driver stage for the latter reason (as an experiment)!!

[fab]

Originally posted by larryg
I understand that this board [19-c] is the newest board used on the dh220/dh500. Does this board have any of the improvements
being talked about?

There is no "stock" boards (DH-200/220/280/500) from Hafler that incorporates the mods discussed in this thread (to my knowledge).

[fab]

[QUOTE]Originally posted by djk
[B]"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.

I found the article. See by yourself the scanned circuit. I believe it is John Linsley Hood design.

[mcp]

There's also an article in Randy Slone's "High Power Audio Amplifier Construction Manual" that is related.

A symmetrical amp with current mirrors at the diff inputs coupled to a darlington VAS is featured in Fig 7.6a.

He then followed up with cascodes in the VAS and lastly the drivers and the lateral mosfets in the current stage are in CF fashion.

[slowhands]

Originally posted by fab



There is no "stock" boards (DH-200/220/280/500) from Hafler that incorporates the mods discussed in this thread (to my knowledge).

The Hafler XL-280 and XL-600 incorporate the higher front-end voltage in their PC-40 driver board (circa 1987). This board uses complementary monolithic JFET duals in the "J. Curl" configuration on input, and the Lender VAS, but no current mirrors.

The Self book really awakened audio amp designers to the crucial importance of input stage balance for minimal distortion, and mirrors are one tool to attain it. So far this thread has pointed out two satisfactory means to implement them in complementary symmetry amps; neither is the method I alluded to earlier. Apparently it is not merely possible, but also old news.

[fab]

Originally posted by mcp
There's also an article in Randy Slone's "High Power Audio Amplifier Construction Manual" that is related.

A symmetrical amp with current mirrors at the diff inputs coupled to a darlington VAS is featured in Fig 7.6a.

He then followed up with cascodes in the VAS and lastly the drivers and the lateral mosfets in the current stage are in CF fashion.

I have checked the diy forum titled "Unstable VAS current in amp from Slone book" and I saw the Stochino-based amplifier which incorporates also current mirrors in symetrical input stage (I already had this schematic but I don't know why I have fogotten about it...) I do not have the book on Slone circuit (does anybody can scan it and post it?)

Originally posted by slowhands


The Hafler XL-280 and XL-600 incorporate the higher front-end voltage in their PC-40 driver board (circa 1987). This board uses complementary monolithic JFET duals in the "J. Curl" configuration on input, and the Lender VAS, but no current mirrors.

The Self book really awakened audio amp designers to the crucial importance of input stage balance for minimal distortion, and mirrors are one tool to attain it. So far this thread has pointed out two satisfactory means to implement them in complementary symmetry amps; neither is the method I alluded to earlier. Apparently it is not merely possible, but also old news.

The XL-280 includes the higher front-end voltage but not a regulated one as I have suggested in this thread. The difference between both options is very different. If we can combine both of them, then it is even better (higher clipping voltage before degradation of performance and more stable DC operating point of transistors. Regarding J-fet at the input pairs, I know Mr Borbely uses J-fet a lot but what are the pros and cons?

What is the method you have "alluded to" earlier for current mirror use in sym input amp?

[fab]

Originally posted by mcp
He then followed up with cascodes in the VAS and lastly the drivers and the lateral mosfets in the current stage are in CF fashion.

The cascode in the VAS is one of the mods I have done in my DH-200 amps and I am pleased with the results. What is the best base voltage for the cascode transistor?
"...current stage are in CF fashion": Slone's circuit required please.

[mcp]

In Slone's amp, cascode base is 4.4V from rails.

Mail me your address and I'll attached a copy of the circuit to you.

Regards