Preamplifier Power Supply Rail Capacitance, How Much? - DH-101 vs DH-110

[Ron AKA]

After 30 years service I'm planning a refurbishment of my Hafler DH-101 preamp. I will replace all the electrolytics. There are two 1,000 uF caps after the bridge, and two 100 uF caps after the voltage regulator on the power supply board. The main TX fuse is a 1/16 amp slow blow, so not a lot a power obviously. The 100 uF struck me as small, and I planned to do a modest increase in value.

As I am doing my power amp too, I have been comparing the later Hafler versions to the early ones I have. The later preamp is the DH-110 which seems to be about 1985 compared to the DH-100 at 1979. I was totally amazed at the difference in design. The diodes are still 1 amp like the DH-101, but 200V instead of 100. The fuse is 1/10 amp instead of 1/16. The after bridge cap is the same at 1,000 uF. But the big difference is that there is really no large capacitance after the regulator in the power supply section. Instead they have put capacitance in the phono section rail, and also in the tone/volume section, and also for both channels. The caps are 470 uF each, and there is a total of 8 caps on the rail for nearly 4,000 uF of capacitance on the board rails!!

Anybody have a theory on why they would essentially go from 200 uF after regulator to 4,000 uF? They put film bypass caps on each electrolytic too. Benefits from all that extra capacitance? A gimmick or of value? Value of independent capacitance on each channel?

If you want to see the schematics they are on line to compare.

Hafler DH-101 Adobe page 12-13

Hafler DH-110 Adobe Page 20-21

[!]

I'd just replace the capacitors with good Japanese low ESR types to keep it working. Even after tweaking these pres won't come close to the performance of something modern using a reasonably good opamp and minimal part count. On the other hand if it is not performance you are after but instead preserving the sonic signature, you might opt for regular ESR capacitors.

You might find a different result but if you make an old discrete design too lively it can start to show faults, sound grainy instead of smooth and slow. Then again I have never heard these two pres, and suggest you go ahead and replace caps as desired then decide if more needs to be done.

[Ron AKA]

Thanks for the response. I really have no expectations of trying to replicate any specific performance level. Over the past 30 years, I have been more than satisfied with the sound, and unfortunately as one ages your ears get worse more quickly than solid state components. Also, the specifications as quoted by Hafler for the newer DH-110 are no better than the DH-101.

Phono THD < 0.0006% (both models same)
High Level THD < 0.001% (both models same)
Noise -86 dB Phono, -90 dB High Level (both models same)

About the only significant difference is the range of tone control. But this is no concern as I bypass the tone control.

My interest is more just curiosity and theory. And, it is obviously very easy and inexpensive to put more capacitance and lower ESR in the same space compared to what could be done 30 years ago. I just have to sort out how much capacitance I want to put in, where it should be located, and how low I should go for ESL. I have some theories but was interested in those of others to see if I'm off in the weeds or not.

[GringoAudio]

Finding this is very timely. I was looking at the schematic for the DH-110 last night briefly and wondered why there was so little capacitance after the regulator. I did not follow the diagram to the individual sections (phono, etc) to see the capacitance in those locations. I am relieved to hear that there is more there. I just picked up a DH-110 pre and DH-220 power amp on ebay and kijiji. Love the sound. The specs say that these should be as good as any modern units.