Borbely Servo 100 amplifier, 25 year project

[GlennW]

Audio Amateur magazine published audio amplifier projects by Erno Borbely in 1982 and 1983. During the late 80s I built many of these. I am still using two of the amplifiers, and gave away several to friends and family that are all still in use. The design has stood the test of time, not a single failure in the years since.

One of the projects was never built and got boxed away. I am not exactly sure when I stocked the original parts, probably early 90s. Good stuff - circuit boards from Old Colony Sound Lab, 500 VA transformer, separate rectifiers and big capacitors for each channel, and Dynaco 150 chassis purchased from Sound Values, liquidator of the Dynaco bankruptcy.

The box came off the shelf in February and was finally put together. After getting it together I powered it up slowly over a day using a variac to give the old capacitors a chance. And it now works very well.
The last orphan is finally functional.


I am wondering how many others are using Borbely amplifiers. Any others out there?

 

[I LIKE MUSIC]

Interesting.

Thanks for the picture.

 

[Alan0354]

Can you post the schematic?

 

[GlennW]

Schematic:


The construction article is still available. Go to http://www.audioxpress.com/, navigate to Shop / Books - Audio and find book "Audio Amateur Power Amp Projects". This book is a collection of projects published between 1970 and 1995. The schematic shows itself as a predessor to Hafler XL280.

 

[Pio1980]

Schematic:
View attachment 942439

The construction article is still available. Go to http://www.audioxpress.com/, navigate to Shop / Books - Audio and find book "Audio Amateur Power Amp Projects". This book is a collection of projects published between 1970 and 1995. The schematic shows itself as a predecessor to Hafler XL280.

I was going to ask about the similarities to the amps he designed for Hafler.

 

[Binkman]

YAY or YEAH or YES!!

Frigging thing with stored parts including.. (old capacitors?) OP just drove several nails into a lot of 'theory'.

Just got bored and?? LOL

 

[GlennW]

Yes, the project was almost all stored parts, including old capacitors. Restart of the project was driven by basement cleanout, and resolving that it was time to either build the project or get rid of the big box of parts. After deciding to build the project, consideration was given to whether it was worth it to build an old design or change to something new. I considered the diyaudio Honey Badger amplifier but the printed circuit boards are too big for the chassis. The price tag of replacement large power supply electrolytic capacitors persuaded me to try the old ones. I did reform the capacitors by powering up over many hours. They seem fine. The only electrolytic caps on the circuit boards are 2 filter caps. The stored ones were good quality with 100 V rating. I did not replace these either.
The only old part that did not check ok was a film capacitor, the kind of part I generally expect to last forever.

 

[Alan0354]

The amp looks to be decent design. Front end IPS is complementary long tail pairs design, output is MOSFET which was very new in the 80s. I think it's worth building. It is not a super high end design, but definitely good enough. You have two pairs of MOSFET output transistor, you can do worst. I've seen some low end Yamaha only has one pair of output transistor and no external heatsink. Your's definitely a big step up.

If money is tight, I would just use the old filter caps. If you have a scope, you can look at the ripple waveform to determine whether the cap is still good enough. But just use it!!!

 

[Binkman]

The last orphan is finally functional.
OP's first post. Using Variac etal. Cheers to OP and the 'gold' finish looks great!. Well you could have sold it as a kit? but now it's probably worth a lot more even if to keep it.

OP knows his stuff... variac power up... not many in wisconsin own one for starters.

 

[Binkman]

Just bumping this up. OP used old stored parts many would have said impossible. So OP is proof in the pudding of what can be done. And he's driving mosfets to boot. nice

 

[Pio1980]

Who is the MOSFET amp guru, as far as you know? I'm thinking of having one gone over.

 

[BuzzK]

Who is the MOSFET amp guru, as far as you know? I'm thinking of having one gone over.

Dave Gillespie sure understands them . . . Don't know what his schedule / intake situation is like.

 

[Pio1980]

I like the Muse enough to have it gone thru by an expert, and upgraded if possible. It may be a forever amp.

 

[BuzzK]

I like the Muse enough to have it gone thru by an expert, and upgraded if possible. It may be a forever amp.

What kind of issues are you having with it? What brand / model?

 

[Pio1980]

It's a later Sound Code Systems Muse One Hundred with 16 Hitachi flat pack MOSFETs, 8 per channel, and no IC servo amp feedback. Fair amount of offset drift before it stabilizes, and the idle current varies greatly between the output devices as do their individual temps. It runs quite warm as is and aspires to sound like a really good ballsy tube amp with considerable "swagger". Offset is sensitive to internal temp, making a bit of a bugger to set to an acceptable offset range.
There was at least one later version with post-Hitachi MOSFETs and IC servo offset correction.
Here's a couple of reviews of earlier versions;
https://www.stereophile.com/solidpoweramps/491muse/index.html

https://www.stereophile.com/content/muse-model-150-monoblock-power-amplifier

 

[Pio1980]

It's a later Sound Code Systems Muse One Hundred with 16 Hitachi flat pack MOSFETs, 8 per channel, and no IC servo amp feedback. Fair amount of offset drift before it stabilizes, and the idle current varies greatly between the output devices as do their individual temps. It runs quite warm as is and aspires to sound like a really good ballsy tube amp with considerable "swagger". Offset is sensitive to internal temp, making a bit of a bugger to set to an acceptable offset range.
There was at least one later version with post-Hitachi MOSFETs and IC servo offset correction.
Here's a couple of reviews of earlier versions;
https://www.stereophile.com/solidpoweramps/491muse/index.html

https://www.stereophile.com/content/muse-model-150-monoblock-power-amplifier

Internals;

 

[Pio1980]

Sorry for the hijack, it was an opportunity to ask something of MOSFET amp expertise.

 

[BuzzK]

Sorry for the hijack, it was an opportunity to ask something of MOSFET amp expertise.

Congratulations on your successful hijack.

You really need some pretty good background in theory as well as "real life" MOSFET experience to fix and stabilize the amp.

I'm wondering why the differential current-hogging -- there appear to be some low-ohmage current-limiting resistors on each device. I don't see any ferrite beads, which are always a good idea to prevent spurious oscillation with MOSFETS.

Were the devices matched prior to installation? My guess is that they weren't.

I'm sure Dave G. could do it (in terms of capability). I'm not volunteering anyone . . . this will be a somewhat expensive proposition, because it involves both design AND testing.

Good luck!

 

[Pio1980]

Congratulations on your successful hijack.

You really need some pretty good background in theory as well as "real life" MOSFET experience to fix and stabilize the amp.

I'm wondering why the differential current-hogging -- there appear to be some low-ohmage current-limiting resistors on each device. I don't see any ferrite beads, which are always a good idea to prevent spurious oscillation with MOSFETS.

Were the devices matched prior to installation? My guess is that they weren't.

I'm sure Dave G. could do it (in terms of capability). I'm not volunteering anyone . . . this will be a somewhat expensive proposition, because it involves both design AND testing.

Good luck!

Thanx!
It appears to be an evolutionary design process starting with a pretty good pro amp by the original designers. This is obviously a minimalist but effective approach, apparently well-behaved. I think the offset drift is due to lowish global feedback correction, and it's possible the output devices were not matched. It seems like they should be, but it may also be possible the designers may have deemed the additional effort redundant, expecting a "commutating" of the output devices as output demand increases, bringing in more devices with the rising load demand. Its hard to second guess this sort of thing without direct communication with the designers.

 

[Pio1980]

Definitely interested in how this works out, I was an early adopter of the DH-200.