Hafler DH-200/220 Mods

[Dick West]

I forget, is the PC-19 board for the DH-200 or Dh-220?

The output MOSFETs are very rugged and if sanely used in a properly fused amp they very rarely fail. You could always swap them in pairs from one channel to the other to check which ones do and do not work.

Caution, each MOSFET should have a number stenciled on it. These devices must be used in matched pairs (matched P channel devices and matched N channel devices). A matched pair will cost $30+ and if 4 pairs are used in an amp that totals $120. You could purchase another DH-200 for about this cost, use it for spare parts, and sell off the remainder.

The original DH-200 (PC-6 boards?) sounded better with some mods. The DH-220 incorporated most of these mods into its circuit. There is little you can do to mod the DH-220.

Hope this helps.

Dick

[KarlDL]

Excuse me if I travel a path blazed long before, as I'm a newbie and haven't searched all posts historically.

A year ago, I began the "HiPer Hafler" project on my then 22-year-old DH-220. The salient aspects of my mods are:

a) Musical Concepts PA-3B driver boards
b) Plitron/Musical Concepts toroidal power transformer
c) Dual-mono power supplies
d) 48,000uF per channel PS capacitance
e) IXYS FRED diode FWBs in each channel
f) A step-start (!) circuit on the AC input
g) Total rework of ground/common wiring

The step-start is probably the one feature that's not likely to be found elsewhere. It comes from my many years working with high-power radio and TV transmitters, where such circuits are commonplace. The essence is simple: there's a resistor in series with the AC connection to the toroidal power transformer input and a relay shorting that resistor. The relay closes when PS voltage approaches normal, thereby limiting inrush current in the PS. The lights don't dim, the power switch doesn't fail, and the "step" (i.e., relay closure) occurs in less than a second from initial application of power. And there's much less "thump".

This was a fun project. Did the amp sound better? Yes. Does it equaly my Odyssey Stratos Extreme? No. Would I do it again? Yes!

The overall clarity of the amplifier improved, along with its ability to handle LF transients. It doesn't equal the Odyssey on the bottom end, but the latter amp has twice the PS capacitance, so that result's not surprising. My general impression is that it has a slight edge over the stereo Odyssey on midrange and highs. But that advantage diminishes with the Odyssey in its Mono Extreme configuration.

I'm looking forward to another 20 years of good service from this amp, albeit in my second system.

What would be interesting, sonically, is to operate 2 DH-220s, each as a mono, with one channel with input shorted, sinking the return lead from the speaker driven by the other channel; i.e., in an unbalanced bridge mode. Has anyone tried this?

[Davet]

Originally posted by Dick West
I forget, is the PC-19 board for the DH-200 or Dh-220?

The PC-19 boards are those found in a DH-220. I noted that they still have mylar caps and carbon resistors on the PC-19 board. Marsh and Jung replaced their resistors with MOF. I have heard the noise reduction of Metal Film over carbon resitors in other projects. They also suggested the use of polystyrene caps. Sound wise, in the signal path polysyrene should sound better than mylar caps.

The original DH-200 (PC-6 boards?) sounded better with some mods. The DH-220 incorporated most of these mods into its circuit. There is little you can do to mod the DH-220.

I have not set down and compared the circuits to determine what mods if any were included in the PC-19 board over the PC-6. With the exception of the components listed above are there any reasons not to "upgrade" these caps and resistors on the PC-19 board?

I have heard improvements in the sound that comes from my modified PC-6 Board DH-200. The Marsh and Jung Pooge article from The Audio Amateur magazine made extensive mods to the DH-200 and they documented the results they heard. I have heard with my own ears that the unit can sound better after some mods. The Pooge and Muscial Concepts mods as well as this thread lead me to believe that the DH-200 can be satisfactorilly modified.

Is there something that I am overlooking regarding modifying the DH-200?

[Thomas001]

Hi Karl,

"there's a resistor in series with the AC connection to the toroidal power transformer input and a relay shorting that resistor. The relay closes when PS voltage approaches normal, thereby limiting inrush current in the PS. The lights don't dim, the power switch doesn't fail, and the "step" (i.e., relay closure) occurs in less than a second from initial application of power. And there's much less "thump".

Can you give more detail as to the resister value and relay specification, and how big is this add-on module? Do you have a schemetic for it? Thanks in advance.

Regards,
Tom

[fab]

Regarding mods, has someone tried to remove the series output choke in the DH-200/220 amps?

Here I make reference to the Graham Maynard's article on Class-A imagineering in Electronicx world magazine". Graham seems to claim that the output inductor increase the loudspeaker induced amplifier distortion.

[Elso Kwak]

Originally posted by Thomas001
Hi Karl,

"there's a resistor in series with the AC connection to the toroidal power transformer input and a relay shorting that resistor. The relay closes when PS voltage approaches normal, thereby limiting inrush current in the PS. The lights don't dim, the power switch doesn't fail, and the "step" (i.e., relay closure) occurs in less than a second from initial application of power. And there's much less "thump".

Can you give more detail as to the resister value and relay specification, and how big is this add-on module? Do you have a schemetic for it? Thanks in advance.

Regards,
Tom

Hi Tom,
Must be something like this:

[KarlDL]

This responds to Tom's query regarding my step-start circuit.

I used a 110VDC relay, Potter & Brumfield K10P-11D15-110, connected across one of the two "quasi-mono" power supplies via a 2K ½W series resistor, with its contacts (2 sets in parallel) shorting out a 5 Ohm, 50W resistor which is in series with the 120VAC feed to the toroidal power transformer primary.

When the amp is first turned on, the 5 Ohm resistor limits the instantaneous inrush current (and, in essence, the charging current through the PS caps) to 24A (12A per supply). This keeps the FRED bridges well within their ratings and reduces overall stresses.

Once the power supply charges up to around 110V or so, the relay closes and places the amp in the "run" mode. If I wanted to use speaker-delay relays, I would slave these to the closing of this "second step" relay, if not using a separate 5-sec time delay circuit.

The sketch provided by Elso is not what I did, but it should also work. It's based on the dynamic impedance of the power supply loading down an AC relay and preventing its coil from having sufficient voltage until the supply is sufficiently charged.

The 50W rating on the resistor is somewhat arbitrary. One can probably get by with less, but I've not performed the integration calculation necessary to more precisely define what's needed.

In the attached picture, the resistor and relay are seen in the upper right-hand corner.

Karl

[Thomas001]

Thanks for the help from Elso and Karl. BTW Karl, what kind of power caps are those that enable you to squeeze 8 of those in there.

Tom

[KarlDL]

The caps are CDE 12,000µF 80V, acquired from Parts Connexion, not far from you. Chris had these on closeout a year ago. The layout is rather tight and I'm still not sure how I got the clamps to fit. I'm in real trouble if one of the "interior" ones ever spills its guts.

[Thomas001]

Hi Karl,

Sorry to border you again, but I really your help in building the relay using the recommended K10P-11D15-110. As per the K10 series wiring diagram (http://www.carlton-bates.com/Current_Catalog/169.pdf), can you please tell me how to connect the incoming AC, the 2k paraell and the 5 R resistor and the AC output. You help in this matter is much appreciated.

Regards,
Tom