Question about Paradigm PreAmp, and DC Input toDH-200 & Hsu Sub

[DrDan]

I need assistance with a question about a new (to me) Paradigm preamplifier and whether its direct coupled output can be used with an unmodified Hafler DH-200 or with a Hsu Research VTF-2 subwoofer without potentially damaging the amp or speakers.

I have scanned several threads on modifications and tweaks for the Hafler, and I have the original manual and schematics, but can't read them to know if there are input capacitors or a servo to handle DC input. I built the amp from a kit over Christmas, in '79 or '80.) I've also searched the Hsu website without success for the same kind of information on this question.

I've enjoyed reading some of the threads with members like Dick West who know a great deal about the Hafler.

Any help would be appreciated. I do have much to learn. (My first step will be replacing the original output capacitors in the Hafler, which I just ordered this afternoon.)

Thanks

Dan

[Dick West]

Thanks for the kind comments.

Do you have a VOM? Have you measured the DC offset of your preamp?

The DH-200 has an input cap that, among other things, blocks DC from getting into the amp, DC that may come from the preamp. This is pretty much standard practice.

As long as C1, the 10 mF non-polarized cap, is on the circuit card and functioning
you will be OK to use your preamp.

Hope this helps. It is good to be safe rather than sorry.

But, here is a possible problem. Have you measured the DC offset of your DH-200? These old amps have no DC nulling circuit and many, over the years, have come to have DC offset problems. The first 4 input transistors, in the differential amp circuit, must be matched to each other on hFe. Sometimes these transistors drift out of spec or get damaged by too high input potentials. If so, you could have DC offset at the output of your amp.

Let us know about your DC offset readings of both the preamp and amp outputs.

[DrDan]

Just completed the testing suggested, and found that the DC offset at the preamp outputs were measured as 0.0 mV's, right and left.

With the preamp outputs connected to the DH-200, the DC offset measurements for the amp outputs were:

-54.2 mV's on the right channel
-11.7 mV's on the left channel

The MM test leads were connected red to red, black to black on DH-200 outputs, and red to inner lead and black to outer ground on RCA pre-outs.

I went back to the Hafler DH-200 kit directions and learned that the input transistors, which can be causing the DC offset, are not identified as such in the materials.

I am interested in learning how far "off" these numbers are, and what the next step should be.

Thanks!

Dan

[Dick West]

The amount of DC offset you measured is low enough to not cause any major problems. Don't worry about it. Some "experts" say that anything less than 100 mV DC offset is OK. Personally, I like to keep it below 50 mV. I have measured some old DH-200 amps and found they only have 10-15 mV of DC offset. Your readings are acceptable.

The transistors that can cause DC offset are Q1 and Q2, and Q5 and Q6. Q1 and Q2 should match each other within 10% on hFe. Ditto for Q5 and Q6. The Hafler manual for the DH-200 did not comment on all of this because it was not supposed to happen, the modules were built and tested at the factory. However, over the years, some of these 4 input transistors may drift out of spec and because they no longer match DC offset will occur. This is a frequent problem with these old DH-200 amplifiers.

The DH-220 included a little circuit to null DC offset. The DH-200 does not have this circuit.

Anyway, it seems you are OK with your project. Good luck.

Dick

[DrDan]

Dick,

Thanks! I plan to replace the output capacitors with 27K uF's as the first "mod"/upgrade.

Dan

[Dick West]

I would not call them "output" capacitors. They are power supply filter capacitors. Before you install these larger value capacitors you might want to read some of the threads here about "inrush current limiting." The power switch on your amp may not withstand the added demands of inrush current required to charge the larger caps you intend to install. You may find your ON/OFF switch will burn out or its contacts may get welded together. Later versions of the DH-200 and other Haflers used a huskier switch. Good luck.

[DrDan]

Dick,
Thanks for the information. I had the power switch replaced about ten years ago though I can't say whether it will be up to the demands of the situation you describe. I'll do the search you suggest and see what may need to be done.
Dan

[DrDan]

Just to be sure, the attached pic shows the kind of device we're discussing, an inrush current limiter (thermistor). Dan

[DrDan]

So the next question for this DIY newbie is, where does one insert the 2 ohm inrush current limiter into the AC mains circuit in the 220? Can it be placed in parallel with the 5 amp slo blo fuse? Should it be inserted into the other lead that runs to the power switch? Is there some way to solder the item that involves the power switch? All help is appreciated.

Thanks
Dan

[Dick West]

Your amp has two Thermal Breaker switches, one attached to each of the two heat sinks. If you trace the wiring to these two switches you can discern that it is really a "loop" of the incoming AC supply. Just unsolder one of the two wires that goes to one of the thermal breakers. Then, insert the thermistor in series with the AC by soldering one end of the thermistor to the now vacant terminal on the thermal breaker and solder its other end to the AC wire freed when it was unsoldered. This will put the Thermistor in series with the incoming AC and by doing the wiring as described you can leave the thermistor hanging free in the air so it can cool as best possible.

Easier to do than describe. Keep your connections as short as possible but don't try to cover them with heat shrink tubing. The thermistor will get hot.