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Old 1st June 2007, 06:30 AM   #1
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Join Date: Oct 2006
Default Hafler DH-101 Preamp

I know this is an old question about an old preamp - but I could use some help. My son just came back from "dumpster diving" and came up with an early model Hafler DH-101 preamp - serial # 1108025. I once own such a beast and performed a number of modifications upon it back in the 80's. However, I have forgotten everything!!

Here are my questions:

1) What did Hafler's DH-106 modification kit for the DH-101 consist of? Wasn't it related to swapping out transistors?? Do you recall the part numbers and what was swapped for what?

2) TMy son's unit has transistors with numbers like MPS13, MPS63, BC559, BC549c, BC415 and BC414. However, I tracked down a manual/schematic from Rockford's Hafler site (for a unit w/ serial # 1812829), and the parts list for the unit shows transistor numbers ALL starting with "MPS-A", like MPS-A-13, 18 and 6523. Which unit (mine or the 1812829 unit) has the updated transistors??

Any help is appreciated!!

Pete Snyder
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Old 1st June 2007, 06:41 AM   #2
jaycee is offline jaycee  United Kingdom
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Location: Norwich, UK
I'd say the MPSA devices are the newer ones.
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Old 1st June 2007, 11:54 AM   #3
YFW is offline YFW  United States
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Join Date: Jun 2007
MPSA13/63 is correct,they are darling transisitor and used in the output stage.In DH110,it use two transisotors to replace the darlington pair.

Now we have a lot of transisotrs to upgrade the circuit.For example,BC550C/560C or MPS8099/8599 are all good choice.
If possible,you could try to use the matched transistors in DH101,it really helps because of it's all push-pull structure.

DH101 offers an excellent sound quality with soft timbre.Enjoy It!
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Old 1st June 2007, 03:47 PM   #4
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Join Date: May 2003
Location: SF
Default DH106 specs

The DH106 kit updates the early DH101 preamps
to the then current production parts list. It is
designed for all DH101 with Serial numbers
1918999 and below. Eight parts as follows.

For each channel:

The original Q1 is MPS-A-18, replaced by NPN BC413C
The original Q2 is MPS-A-6523, replaced by PNP BC415C
The original R10, R11 are 100R 1/2W replaced by 47R.

Q1/2 are the transistors on the far left as viewed from
the front of the DH101.

I have several paper copies of the DH101 manual
and looks like they were not updated to include
the DH106 modification.

A completely original DH101 will have only MPS
designated transistors.

I bought the DH106 kit in December 1979 so the kit's
availability dates from sometime in 1979 and was
no longer listed on their 1983 pricelist (probably
due to the DH110 superceding the DH101).

Incidentally, if you replace all the carbon film resistors
with metal films, you will be blown away with the
sound - spooky black and best dynamics of the
entire Hafler family. Then change out all the caps
to PP caps, update the power supply and you
will enjoy this unit for another 20 years.

All Hafler preamps following the DH101 used metal
film resistors but none of them sound anywhere
close to the DH101 with them.

Enjoy the music

PS If you have other than the above parts in your unit,
please post. It may be that they were used at the
very tail-end of DH101 production and they never
bothered to issue an update kit.
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Old 1st June 2007, 05:42 PM   #5
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Join Date: Oct 2006
From what CaliforniaBob says, it is apparent my unit had the DH-106 changes made. My unit has the BC-414 and 415 transistors at the far left side of the circuit board - plus there is a mixture of metal film and carbon film resistors. So, I would say this unit received the upgrade.

Now, having figured this part out (with everyone's help!), I would also ask about a modification I seem to remember Musical Concepts sold back in the early 1980's that had to do with the powesupply - wiring, improving the grounding scheme and maybe even something about the voltage regulators themselves. Does anyone recall these modifications - or anyone else's that tackled powersupply and grounding problems???

Thanks again!
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Old 1st June 2007, 08:23 PM   #6
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Join Date: May 2003
Location: SF
You could simply add a little more capacitance before and after
the regulators. also, replace the diodes with fast recovery or
shottkey diodes. Also, a bypass for the big caps.

I think that the biggest bang, in terms of changes, are
metal film resistors, then cap changes, then power supply.

And!!! Lastly, replace the red LED with a BLUE one!!!

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Old 2nd June 2007, 05:27 AM   #7
YFW is offline YFW  United States
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Join Date: Jun 2007
If possible,try to replace the regulator and capacitors.
The shunt regulator have fast response in DH101/110 preamplifiers.
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Old 1st February 2008, 05:32 AM   #8
poko is offline poko  United States
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Join Date: Aug 2007
I have 3 Hafler DH101 and I'm looking to replace/update the transistors. I was able to find most at Mouser, but I'm having trouble finding a supplier for BC415C. I think it's even harder to find the MPS-A6523 that was originally specified, but I figured the BC415C would be better being that it was part of the DH106 upgrade.

The original MPS-A18 was replaced with BC413C in the DH106 upgrade, but the newest of the factory-assembled DH101 that I have used BC414C instead. I'm having trouble finding that one too, but I was able to get MPS-A18 so I can live with that.

Anyone know any sources for BC415C or MPS-A6523?

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Old 20th September 2011, 10:37 PM   #9
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Join Date: Oct 2007
I am bringing this thread back to life because of all of the improvements in electronic components. I have to pull a Phase Linear 3300 out of my rack and reinstall my Hafler DH-101. I want to upgrade this mighty little pre.Any suggestions?


Last edited by speakerman1; 20th September 2011 at 10:37 PM. Reason: spelling
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Old 20th September 2011, 11:43 PM   #10
Ron AKA is offline Ron AKA  Canada
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Join Date: Aug 2011
Originally Posted by speakerman1 View Post
I am bringing this thread back to life because of all of the improvements in electronic components. I have to pull a Phase Linear 3300 out of my rack and reinstall my Hafler DH-101. I want to upgrade this mighty little pre.Any suggestions?
I'm doing the planning to refurbish mine. I will replace all the electrolytic caps, just because they are 30 years old. The output coupling caps in the phono section and high level are 10 uF electrolytics. I will replace with metalized polycarbonate film caps. Same with the tantalum 10 uF input couplers - replace with 10 uF metalized polycarbonate film.

Still a bit undecided about the caps to use in the power supply. See this thread which has not gotten much of a response. I will probably go to 35V 1500 uF Panasonic FR for the pre regulator, and perhaps 680 uF FR series after. The idea is to get ESR as low as is reasonable. Because of the increased capacitance, I will likely replace the diodes with 2 amp 200 volt (1 amp 100 volt now). Do not plan to use fast recovery, as I concluded from this thread that there is no advantage. See page 8 or so, where some actual scope traces are shown after much conjecture as to what really happens. Will put a 0.47 uF film across the large main filter caps to kill any diode noise. Will also probably put another pair on the board power rails (if I can find a place to fit it). Will put some 0.1 uF X7R ceramics on the AC feed to the diode bride to reduce any noise on the AC. Suspect I will have to change main fuse to 1/10 amp from 1/16. It has blown once already, and with more capacitance is not likely to be reliable at 1/16th.

Still undecided, but will probably replace the signal circuit resistors with metal film. They are currently carbon film. The DH-110 uses metal film, but surprisingly the specs don't show the noise is any lower, so still wondering about the real value...

If I can find gold plated input/output jacks that will easily retrofit, I will replace them.

Last I will probably put a 10 amp relay in the power supply area to feed the switched 110 volt outlets. I start my power amp (DH-200) using the switched outlet and the main power switch is probably suffering from the practice.

I think that is about it... Is there anything left but the name on the front???
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