Hafler DH101 preamp repair and mods? - Page 2 - diyAudio
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Old 15th November 2006, 01:02 AM   #11
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I want to upgrade the resistors in the 101 with metal film. The original carbon film resistors are 1/2 watt but I have a supply of 1/8 watt metal film. Any thoughts on whether the 1/8 watt resistors would create issues? Thanks.
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Old 15th November 2006, 02:33 AM   #12
gni is offline gni  United States
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Default 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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Old 15th November 2006, 03:00 AM   #13
anatech is offline anatech  Canada
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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
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Old 15th November 2006, 03:24 AM   #14
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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
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Old 15th November 2006, 12:14 PM   #15
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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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Old 30th November 2006, 12:07 PM   #16
eriwer is offline eriwer  Sweden
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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.
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Old 30th November 2006, 05:12 PM   #17
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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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Old 1st December 2006, 12:37 AM   #18
eriwer is offline eriwer  Sweden
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Thanks for the tips jackinnj. You're right, I was talking about C11 and C20.

So next I'll try the star grounding, look for som EMI filter and replace the pots for carbon resistors. I'll be back when it's done.
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Old 1st December 2006, 12:54 AM   #19
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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
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Old 1st December 2006, 09:07 AM   #20
d3imlay is offline d3imlay  United States
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During the production run of this preamp the demand for 3 pin voltage regulators exceded the supply. During this time Hafler used an opamp based voltage regulator which was highly regarded. Are you fortunate enough to have that version?
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