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mjatabor 6th March 2013 02:55 PM

Hafler 915 preamp cap question
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I have a early '90s Hafler 915 preamp in perfect working order. It is smooth and musical, but not the most detailed. Anyway, I like its sound very much in my system.

Would I benefit with replacing the two 4700 mfd 35v polarized electrolytic caps in the power supply (large black caps in upper left of photo) to something better? What would your recommend (they are short and wide)?

RNMarsh 7th March 2013 05:18 AM

depends... what brand are they? can you show other side of caps in question. -try to find ones which have lowest Z at highest frequency. -- RNMarsh

mjatabor 7th March 2013 11:47 AM

Thanks Mr Marsh,
I was too lazy to open it up to take better photos so I found one on the web. But, the caps are a non-specific brand?
What do you mean by 'low Z'.
All I find are electrolytic in my search. Is there a better option, or is it ok to get a better electrolytic cap?
Lastly, should I up the mfd to a higher value than 4700 mfd, 35 volt?

Mooly 8th March 2013 12:00 PM

Most important is to get caps that physically fit so check the data sheets of any you are looking at for size and pin spacing.

Low Z or low E.S.R. denotes a low internal impedance. That is perhaps more applicable to switching type supplies rather than linear ones such as this. I wouldn't increase the value at all and yes, any caps will be electrolytic types. Go for a reputable brand rather than a "generic" unspecified device.

(I suspect there is probably no benefit to be obtained by swapping these)

mjatabor 8th March 2013 01:19 PM


Originally Posted by Mooly (
(I suspect there is probably no benefit to be obtained by swapping these)

Thanks Mooly,
I was wondering about this. Musical Design, who in part helped design this preamp of which their caps are used in it, also makes a mod for it. But the basic mod costs 399.00 and he is not volunteering what he does so I can perform the mod myself. I guessed the power supply caps where part of the mod, hence my interest in swapping 'em out, but I'm not anxious to do extra work for nothing.

mjatabor 9th March 2013 07:02 PM

1 Attachment(s)
Ok, I'm back with one more question. Here is a photo of my actual preamp (earlier photo is not my unit).

I now see that mine has not been modded like the first photo posted earlier. The 8 other yellow Musical Concept caps populating the other 915 preamp are .1 mfd 100v polypropylene caps (8 of them). I figure if I replace these I may have one of John Hillig's mods to my preamp. He is not willing to sell the parts only and I don't want to invest the $399 he asks for his basic mod. What brand would you recommend to use to make the upgrade?

Mooly 10th March 2013 07:43 AM

I'm afraid I'm not a believer when it come to "boutique" parts so can't really advise you on that one :D

Those type of caps don't really deteriorate or change like electros do.

mjatabor 10th March 2013 06:50 PM

Interesting, on this site folks talk about the advantages of better caps.

Mooly 10th March 2013 07:03 PM


Originally Posted by mjatabor (
Interesting, on this site folks talk about the advantages of better caps.

Indeed, and resistors too. The debates run and run. Me... I'm content to choose and use good quality "commercial" grade parts of known provenance. What I look for in terms of sound quality comes more from the circuit topologies I prefer to use.

Hey :D but don't let what I say or think stop you from experimenting.

Ken Newton 16th March 2013 02:27 PM

had a similar experience with my Forte 44 discrete JFET preamp. The Forte 44 is a discrete JFET linestage designed and built by a now defunct spin-off company of Threshold. I thought the uit sounded pretty, until I completely removed it from the signal chain by connecting my low output level home brew DAC directly to my power amp. Wow, what an improvement! The sound is now SO MUCH better that, for now, I've forgone the connivence of even a volume control.

In looking at the 915 schematic, IMHO, Hafler has made generous use of quality JFETs, and simple yet smart circuit discrete topologies. They only places they appear to have taken the easy route is for the voltage regulators, and the headphone amplifier, both of which are IC implementations. I'm not certain about where you would obtain the improvement you seek. Perhaps, the capacitors, or perhaps the volume controls, or perhaps the power supply implementation, or perhaps somewhere else. Sometimes, it's less of a headache to simply obtain a different, more satisfactory sounding component.

If you do not already possess a copy of it, here is the link to the 915's service manual which includes schematics. Courtesy of, 'mlloyd1'.

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