Hafler DH101 preamp repair and mods?

I made a couple of things on HaflerDH-101, put premium rca connectors, replaced the transistors, put audio capacitors,put mf resistors, changed the complete wiring, I plan to put another diodes for probe, in there are 1n4002 1a 1000v 4 of them, so i'm interested in what do you recommend and what values (putting uf4007, hexfred, schottky or other? ..)Thanks
 
FWIW, here's the rest of the DH-101 articles/letters/comments from TAA.

View attachment 693184
Unless the transformer is over-heating, there is no necessity of using two transformers.

Philpot is (was) incorrect in his thinking that two transformers are better than one -- the impedance of the regulated power supply will be dominated by the regulator and pass transistor.
 
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Hi Jack,
I couldn't agree with you more!

I recently had to make a modified unit reliable (Musical Design or Musical Fidelity). That "upgrade" deleted the buffered output section, ran the parts very hot and used two oversized power supplies. One for each channel. It was a disaster and cost the owner a lot of money. It only had one supply when I was done with it, and other changes not only made it reliable, but it also sounded a lot better too. Not all changes are upgrades. The rectifier changes aren't for example.

-Chris
 
I recently purchased a Hafler DH-101 in very good working condition. I've started making some changes and I plan on defeating the tone circuit.

I was thinking of converting the Bass control to an adjustable feedback loop, my thoughts are to remove C15 and C16, then disconnect the wires at 6 and 7. If I leave the pot in the circuit, would this pot allow me to vary the amount of feedback?

Any thoughts?

Regards

Bonanza
 
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Hi Bonanza,
I highly recommend that you leave it stock. The gain isn't so high that variable feedback would work well, and I've seen time and time again that given a solid design, more feedback is really a lot better than less.

If you really want to do some positive things, try using some metal film resistors to set gain and in the input stage. If you can match transistors, that helps as well. Removing the tone control circuit is a big mistake.

Now, if you really want a preamp without tone, build one. They are more simple to construct without the tone stage. You absolutely do want the output buffer stages.

-Chris
 
Despite the commonly accepted idea that tone controls produce degraded sound (many cheap ones do, unfortunately) and that a dsp (digital signal processor) is the ultimate 'new' tone control, this reasonably priced preamp has quite reasonable quality tone controls and quite easy to use

As Chris mentioned, if you want to 'upgrade' the preamp you could also consider upgrading some of the components in the tone control area, but this isn't all that easy to do as 'changing the sound' doesn't automatically create a 'better sound' - I found out the hard way!

If you listen to a wide variety of music with quite different recorded quality (particularly some of the earlier jazz, country, R&R music) simple tone controls are extremely useful, and are a real bonus if you have a good 'headphone setup'
 
Gentlemen,

Thanks for all the great replies. My main reason for removing the tone circuits are:

1. I never use them. I listen to the recording as recorded.

2. I wanted to remove the tone switch and wiring to help improve the overall sound. I might remove the switch and jumper the tone control active at all times or mount relays under PC4 to activate the tone circuit and use the existing tone switch to trigger the relay.

Opinions please.

So far I have removed all wiring to the input selector switches, mono switch, dubbing switch and the ext. patch. I currently have one input wired directly to PC4. It is amazing how just cleaning up the input wiring made quite a large difference in sound. The sound was probably being degraded by the old switches.

I will be installing a 4 input relay input board mounted by the input RCA's that will be activated by the existing push buttons.

I have also disabled the phono section(I use a separate unit) hoping to have some additional current available for the line stage.

The PSU was rebuilt with new caps.

I also removed and jumpered C11 for direct input coupling. My D/A converter and phono stage have output DC blocking caps.

Replaced C13 & C14 with Sprague Orange Drops(I had them in stock)


My next changes are as follows:

Replace C20 with high quality film caps.
Replace resistors with 1% metal film types.
Replace C12 with a 1% Silver Mica
And if I keep the tone circuit, I’ll replace all the caps.

Any additional advise would be appreciated.

BTW - I also love that commercial for the dog bowl, made by the same company that does the laser measured auto floor mats.

Thx
 
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Hi Bonanza,
This is a pretty simple preamp, really basic. You will probably sell it and move into a better unit at some point in the future. I would:
1) Keep the tone stage
2) Restore the phono stage
3) go ahead and install the relay selector.

The input selector and the other switches needed a cleaning. I can understand why you eliminated them from the signal path. As I said, this was a nice but very basic preamplifier. For the money it was a great little preamp. But, if you want to really improve it you are better off starting from scratch. Either with a different make and model of preamp, or a DIY effort.

I typically don't use tone controls either, but there are times when they are mighty nice to have. Plus, because there is a level drop, normally around 10 ~ 20 dB. So there is that extra gain built into the circuit to compensate. That will still be there if you eliminate the tone controls. I'm talking on average here. I didn't specifically look at this schematic again.

Not all resistors will benefit from a change to metal film, although can't hurt either. Same goes for the capacitors. Polystyrene or polypropylene capacitors might be a better choice than the Mica ones you have your eye on.

-Chris
 
Thanks for the great advise Chris....

Just for testing, I removed the wires going to the tone switch and jumpered between 5 and 8 and left 7 and 6 open to simulate the tone switch in the on position and all my a/c hum is gone, I suspect the tone switch is picking up 60 cycle hum from the power switch next to it. I think I'm going to mount 2 relays under PC4 and use the tone switch to trigger the contacts.

BTW, I do understand that this was a budget preamp in it's day, and it's not my only preamp. I just want to see what sonic improvements can be had by updating this preamp with modern components and building techniques.

I think for what these units can be purchased for and with some money spent on quality parts and time invested, these are very good value.

Bonanza
 
A bit of an aside from your project Bonanza

Abit of a 'side-note' for those that do like tone controls at a reasonable price, Shiit Audio have produced a simple 4 band equalized box called Loki 1, for a grand total of U$149 - it's a very transparent and neutral little unit with very useful bandwidth centres and can make a huge difference to the sound of some of the 'deficient' mixes and also just a bit of fun to play with - nothing like the cheap ones of years ago.

It's good that you're enjoying the Hafler preamp project. I once did a similar thing with a NAD integrated amp and was quite surprised just how good it ended up.
 
Yes, it's fascinating how their product line has developed and many of their items are 'relatively cheap' (not all, mind you!) and that they have also included a number of headphone listening products

It's an area that's seen rapid product development in the last few years and some of the products reflect the influence of 'pro-audio' too, unlike the 'hifi scene', altho this is slowly changing too.

Mind you, some of the prices in the headphone area are totally mind boggling too.
 
Just finished my second DH101. The previous one only had the psp rebuilt and a recp of the electrolytics.
On this one I stripped the main board, installed metal film resistor, all new capacitors (mostly polypropylene), rebuilt the psp, upgraded all of the transistors to the equivalent of a "106", replaced all signal wiring with "Radial" studio wire. I also replaced the rca connector patch panels with ones from Musical Concepts.
Got rid of the tantalums and replaced C2 tants with polypropylene, replaced C11 tants with MKL Klanghelms.
I'm also going to replace C20, C13, and C14 with polypropylenes I found some Wima's that will fit C13, C14, and I can squeeze some other poly's in above them for C20.
The preamp sounds noticeably better than the first one which I also thought sounded really good. This is just that much better.
Until I turned it on and ran a signal through it for the first time following the rebuild, I had no idea if it would even work. Having a second unit that was working properly to refer to along the way was a huge help.
 

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JF80,

I've been reseraching the needed transistors for my 101. I have an early version with the MPS transistors, all are NLA as are some recommended replacements.

I see equivalents listed, I guess I could just pick some, but maybe this is a bit out of my experience.

If possible, want replacements did you use? I also need to do the 106 update, so probably need some metal film resistors.

Thanks.

Mike