ideas for holiday rebuild of APT 1 power amp?

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Some years ago , I lost my APT 1 power amp to a severe lightning strike in my apartment. Badly damaged: all output devices fried, most other transistors and some of the resistors as well. I made some inquiries about getting it repaired and trying to locate replacement blank or semi-stuffed PCBs but no luck, so in the corner it went. Did anyone take ownership of the stock of parts, etc. for APT when they went under? Usually, when a company goes belly-up, one of the designers takes the leftovers and at least offers servicing and updates, etc.

Anyways, I found the amp while doing some cleaning a short time ago and decided maybe over the coming holiday, I'd redo the inards myself. So, any good ideas or schematic suggestions from you folks? My caveats are:
1. I want to use the same chassis and not do (much if any) mechanical/metal work
2. The power supply appears to be fine, so I'd like to use it
3. The chassis houses moderate heatsinking for four TO-3 devices. I'd like to continue using that, so Class A is out of the question (I'm saving my "special" heat sinks for my Class A beast in the basement waiting for the "killer schematic" when I settle on it someday :)
4. I've got enough matched pairs of TO-3 package devices: Hitachi MOSFETs (2SK135/2SJ50) or Motorola Bipolar Transistors MJ21193/MJ21194 to use either as output devices. My last few design have been MOSFET based, so I'm tempted to try bipolar to be a little different this time.
5. There isn't enough space to force fit a pair of Leach Low TIM amp boards (At least I haven't figured out a way yet!). That was my first idea to get something good running quickly with minimal effort. I'm just looking for a quick project to whet my appetite over the coming Holidays - a couple of well deserved weeks off (I won't even have any honey-do list items!). I'm not looking for anything like the 1.5 years I spent building and tuning (aka crafting) my Servo 50.

Thanks in advance!
i am more than happy to help you out with this .....

just need to know a few more details 1st.....

what do u expect out of this amp? ie. output power and load and distortion figures

what are the internal measurements available for the pcbs?

what is the voltage of the power supply?

i would suggest a bipolar output stage as you wont be able to bias the mosfets high enough for audiophile sound.
Thanks AudioFreak:
I am expecting at least 100W into 4ohms (KEF104/2 Speakers)
I expect distortion less than 0.2% into 4ohms across 20Hz to 20KHz at rated output (not clipping yet)
Can't get to the chassis at the moment (leaving to visit inlaws for Thanksgiving tomorrow), but something like 3"x10" each side (actually pretty good for direct connecting the output devices directly to the PCB!)
I don't remember the rails exactly - something like +/- 45 (high current for output devices) and simultaneously +/- 55 for the driver supply (nice!). It also has this funky load switch that drops the rail voltage (switches transformer tap on primary side) for 4ohm loads. The design includes a monitor circuit that flashes a red to let you know it's a good idea to flip the load switch for a lower load (also nice!). There was a really nice writeup by Tom Holman in Electronic Design in the early 80's (maybe 83?) back when they used to do audio specific articles. I remember the issue because it contained two audio articles, billed as a shootout between bipolars and MOSFETs. They had an apps guy from Hitachi write about the ubiquitos lateral MOSFETs. They also talked about the design of the DH200 which was pretty popular around that time. Tom talked about the new (at that time) Motorola power devices MJ15023/24 (I think?) as applied in the APT 1 power amp. Nice circuit with the baker clamp anti saturation diodes for the 2nd voltage gain stage.

OK, OK back to the here and now....

I agree with you about the bipolar vs. MOSFET bias comment. I actually thought about replacing the heatsinks with something that still fits but has much better cooling capacity (lot more metal and square area) but can't hold TO-3 devices. It would be great for TO3P and TO264 though. I just have to dig through my parts bins to find them. As you can see, I'm still kinda up in the air about things :)
Is this enough for a start?

Ok, I've got a few ideas of projects that should be suitable and will fit your enclosure without much trouble.

But first, I need some more details from you.

I need to know firm figures for the voltage of the output stage supply rails (when loaded).

Are the inputs to the amp balanced or unbalanced?

I would suggest that you change the heatsinks on the chassis to allow at least 4 devices per channel (ie. 8 output devices total)

I'll also need firm measurements on the internal area available for each amp board.

Are you willing to make your own pcb's? If not and you take my earlier advice to change the heatsinks, you could build Project 3a with a minimum of fuss and as a bonus, it doesn't sound half bad either. Just be warned, if you go ahead and do this, while running the amps into your 4ohm speakers, the supply voltage must not exceed +/-35V ...... if it does the amp will not survive ..... of coarse you could eliminate this problem by upgrading to similar but higher power higher voltage components.

The tiered supply probably wont be of much use to you given this is a bjt design........ you may have to alter the supply rails so that the front end gets about the same as the output.

[Edited by AudioFreak on 11-25-2001 at 06:01 PM]

You might not like my suggestion given your comment about the Servo 50 but here it is.........

I had a couple of amps about the same vintage and power as the Apt. Mine also got zapped by lightning. I adapted one of the Borbely driver boards (forget which one now) but kept the bipolar output stage of the old amp. With a minor change to the bias network, it worked fine and sounded good. I made some circuit mods over the years and have only now started to build new amps. I think a driver board for one of the Borbely 100W amps might work well (even a mod of the Servo 50 would work).

If you want to try this route, I'll be glad to give you more details (or advice if you need it).

Good luck however you decide to do this.

Thanks Phil,

Actually, one of the circuits I've been playing with on paper has evolved into something that looks very very much like the Servo 100 from the old "The Audio Amateur" article from some years back.

I really like the baker clamp circuit in the original Apt schematic, so I incorporated that into the design as well. '

I'll publish a schematic or 2 I've been thinking about (as well as provide more feedback on the carcass power capabilities for the other poster) soon.

Hi Michael

Sounds like you and I have followed some similar paths over the years. My original Borbely amp was a Servo 60 that evolved into something very close to Borbely's newer designs but with a BJT output.

The amp I'm working on now uses a DC150 amplifier board (I got the boards from Welborne a few years ago - no longer available) but it will use MOSFET drivers, probably IRF240 and 9240, then a bunch of bipolar output transistors.

I hope you'll be able to post the schematic of your new project, would love to see it. I used to have that old article by Holman. If I still do, I'll have to give it a reread.

Best of luck with the amp.

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