Hafler DH-110 - Caps - Mute Circuit

[Pete B]

I have a Hafler DH-110 that I built from the kit many years ago. I saw and heard it before I bought, but did not like it after building it and seeing what was on the inside. It does have *very* impressive specs. It was designed around the time when poly and low ESR electrolytic caps were coming into use and uses some in the signal path and power supply. However, there are 3 electrolytics in the signal path for each channel of the line stage. The phono section also has electrolytics caps, there are 1000 uF caps as emitter resistor shunts.

Header connectors are used internally and one became intermittent in this unit.

I always thought that the mute circuit was odd, anyone else notice this?

I used this DH-110 on and off over the last 10 years or so and recently it started putting out what seemed to be a full rail pulse on power up. This went away after using it, could be an electrolytic cap that reformed, don't know.

Pete B.

[rrcrain]

Electrolytics in the line stage aren't necessarily bad or evil as it really depends on the overall design of the unit. For comparison only, my Heathkit AA-1800 amps have one electrolytic in the line imput stage per channel. It's a 10uf polarized low voltage cap used to block DC and for impedance matching. I've considered installing a poly cap but strongly suspect the amp would become over detailed. I also wonder why the heck they used a polarized cap thats wired directly to the line inputs.

Your description of the halfer problem sounds like an electrolytic reforming itself.

[Pete B]

The issue I see with the electrolytics caps is their age. I've replaced all in the line stage with inexpensive poly caps.

The mute circuit does not seem to work reliably in one channel, even with new caps. Pulled the mute FETs, might put a relay in there instead.

Pete B.

[Pete B]

Interesting, the mute control circuit is built around an LM393 dual comparator which I am familiar with. Power to pin 8 is Zener regulated at 10 Volts above the ground pin which is connected to the negative 23.2 V supply. The open collector output, is pulled up to the +23.2 V supply which is far beyond the absolute maximum rating of 36V, with a total voltage of 46.4 V across the output transistor.

What a mess.

Pete B.

[rrcrain]

Is the mute circuit simply a time delay circuit for turn-turn off thump protection or is a dynaic circuit that monitors the signal and disconnects the speakers if it detect an over voltage or low frequency condition on the output?

Do you have a schematic for the circuit?

[rrcrain]

Looing at the schematic, it looks like they use the signal from pin one from IC3 to mute the right and left channel. If the mute circuit works on one channel but not the other, I would be looking at Q15 (right channel) or the corresponding FET in the left channel.

Your right, they do use a very "interesting" mute circuit.

http://www.hafler.com/techsupport/pdf/DH-110_preamp_man.pdf

[Pete B]

As you probably noticed it is a timing circuit and does not monitor the output at all. Interesting that they actually sample the AC from the power transformer, probably to obtain a faster time constant than the main PSU. Yes, the FETs have a common gate control signal so the problem was one of the FETs. I pulled both because the relay is going in, don't stock FETs, had a good relay on the shelf.

I actually tried it without the FETs and there were no thumps, just fade to fuzz and distortion as the PSU went down, which is not there with the mute circuit.

Power up/down control circuits are difficult especially when the circuit itself is loosing power. I've seen lots of digital circuits that would not come out of reset reliably.

By the way, did you take a look at the phono section? The amp is complementary but not diff amp, very strange. Even so, the phono section THD is .0006% at 1K, 3V out.

Pete B.

[rrcrain]

I did notice the phono section and my first thought it was an emitter follower design, which I thought was odd. It's been well over 15 years since I repaired electronics so I'm past the rusty stage.

I'll have to look up the THD of the phone stage in the Heathkit AP-1800 I own for comparison as its phono section uses dif amps.