Hafler DH101 preamp repair and mods?

I've just purchased a Hafler DH101 preamp kit build. It is in decent working order, however, there is some "bleeding" of input sources. What I mean by that is that I can hear the aux and tape sources (if they are active) while listening to phono. Can anyone recomend what to look for to cure this problem? Also, i was wondering if anyone could recomend some mods. I read something about a moving coil upgrade, but i'm not sure what that is or how it would benefit me.
 
I used a modified DH-101 for many years, it was a pretty good design.

The best thing to do with it is what I did, pull out the main pcb, slice apart the phono and line stages separately, and build them into a new case with all new controls, wire, capacitors, jacks, and regulated power supply. Of course that's a bit of a major project, but you really do have an excellent preamp when you are done.

The Audio Amateur lists mods in the Feb. 1979 and April 1980 issues. They include; changing the ground wiring to remove crosstalk and ground loops, removing the phono input selector switches by wiring directly from the phono jacks to the pcb phono inputs, removing the tone control circuitry, and correcting the phono RIAA bass response as well as some other minor circuit changes.

I strongly recommend replacing the electrolytic bypass capacitors with something better, and upgrading the resistors would help too. Replacing the power supply makes a huge difference.

I have some extremely poor photocopies from some of the AA articles as well as the original owner's manual with schematics if you need them. DH-110 as well.

I read something about a moving coil upgrade, but i'm not sure what that is or how it would benefit me

That would be for using with a low-output moving coil type cartridge on your turntable, so that you didn't need a step-up amp or transformer.

Take care,
Doug
 
Thanks for the info! Tinkering with this preamp will actually be my first ever amp/preamp project. I thought it might be a good starting point since my last preamp just fizzled out because it needs a lot of work. (HK Citation 17...why am I not fixing it?...) Anyway, the DH101 came with the original manual and schematics, but if it’s not too much trouble, those AA articles it would be greatly appreciated. In fact, ANY other info would be greatly appreciated.

-Jarrett
jarrett23@comcast.net
 
jarrettl said:
Could someone maybe give me some clues as to what type of hook-up wire to use? I don't mind spending $5-$10/foot on some decent wire...and if i'm lucky, could someone maybe rattle off a some places to purchase such stuff (as wire and switches).

I use CAT5 wire for hookup. Inexpensive, high quality and color-coded.

The DH101 was quite well regarded in its day.

http://www.borbelyaudio.com/special_articles.asp
 
Upgrading Resistors

Sounds like the preamp runs with a fair amount of voltage to require
the use of 1/2 watt resistors. If you want to really get serious. . .
replace with 1/2 watt metal film. . .the 1/8 watt resistors are
underpowered! The preamp is might be over designed to give
it a longer service life though!

So. . . .

You will need to test each resistor at all sorts of inputs and outputs
to see how much voltage is across the known resistor value. .
calculate wattage. . . .then you will know if you can replace the
1/2 watt with 1/8 watt. . . but I will bet that most of the resistors
are operating a more than 1/8 watt!

Just my 1/2 watt worth of input

Chris
 
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Hi bcrescen,
Don't do that. Maybe a couple resistors in the signal path near the inputs, but leave the others alone. Metal oxide if you want to play with the others.

Honestly, redoing the capacitors will bring you much more mileage.

-Chris
 
The DH101 voltage rails are +-18V, so I don't believe that the higher wattages are strictly necessary except maybe for the resistors in series with the output transistors of each stage. However, I've always found that higher wattage resistors give better sound than the same resistor in a lower rating, so I wouldn't do it either. If you really want to upgrade the resistors get some higher wattage Vishays or other high quality type.

Later versions of the DH101 had upgraded metal film resistors that were physically smaller that the 1/2 watt originals, but I don't know if they were lower wattage as well.

Take care,
Doug
 
Focus on the capacitors -- those in the RIAA path you might want to replace with polystyrenics --

you can easily clean up the voltage from the LM317/LM337 regulators with the circuit described by Wenzel Associates in their white paper "Finesse Voltage Regulator Noise" -- this will bring the noise down to some tens of nanovolts per root Hertz. http://www.wenzel.com/documents/finesse.html

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I've done the following mods on my DH-101

*Replaced C2 and C11 with polypropene capacitors. A great improvement in my ears.

*Replaced the electrolytic capacitors in the power supply. Just to keep the amp alive until i get a new power supply. Do you have any tips on the supply?

*Changed the RCA-connectors. The old ones was so poor that it was impossible to connect anything to them.

*Rewired with shielded cables. I don't use the RIIA and have only one source so now the input goes directly to connection 4 and the output from connection 9 goes directly to the output connector.

Next will be to get a more quiet power supply.
I wonder if there is a way to disconnect the bass and treble network. If R23, R25 and C18 is removed, will it do the trick or will it produce problems? I'm not very skilled in electronics so I'd be happy for any comments on that.

I'd also be really glad for any articles on modifications to this amp.
 
C1 and C2 are on the phono preamplifier board. You must mean C11 and C20.

Speaking of the phono section -- you could change C1 to a silver mica, there were many suggestions made in the 4/1980 issue of The Audio Amateur for fixing the DH-101's RIAA -- these include changing R5 to 90.5K, changing C5 to 940uF (by paralleling another 470uF), changing C7 to 24nF and adding 68nF to the junction of C7 and C8 to ground. I simulated this and found that 68nF should probably be 10 or 6.8nF, and that reducing the value of C6 to 6.1nF really helped. This is just a simulation, however. YMMV

For the mains input -- if you have a busted computer power supply (like an ATX supply) you can use the EMI filter for the A.C. input. I did this for another Hafler. If you can rig up a Jung Super-Regulator this will really help.

There was also an article in TAA 2/79 in which the author connected all the input grounds and power supply board to a star ground on the rear panel -- this eliminated 120Hz buzzing. It was also suggested that you calibrate the bass and treble controls (rather than eliminate).

RIAA before and after:

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http://czsr_bac.on.neobee.net/DH101Mod.rar
can be handy,even if I didn't do the same things


what I do is:

several little DPDT relays glued between RCAs on back plate
patch field switch array is used just for selecting relays
additional small xformer and rectifier for relays

entire preamp is rewired:
grounding with one central ground point
important-unused inputs are grounded
all signal wires are shielded (but only on input end)
line stage DC coupled all the way

relays are used also for switching tone circ

stabs are just good old 317 and 337 ,in enhanced topology,for better PSSR (nothing else than cap across adj. resistor and few extra diodes)

etc