I am looking for ideas for a DIY amp/pre-amp or integrated.

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DIYPDMOUT

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Joined 2021
#1
In the early 90s I build a power amp (Erno Borbely design) and after completing it found that every once in a while it would oscillate. I didn't want to spend the time to troubleshoot it so a friend bought it, thinking he could fix it. He tried but after a while (believe it or not) gave up and let it oscillate (which was pretty rare). Lately I've decided that I want to give it another go and build my own amp again. I bought the Randy Slone book and thought about using one of his designs but see some people (on this forum) were having trouble getting the designs to work properly. Could I get some suggestions for a great sounding, reliable solid state design that puts out 20 to 60 watts. Especially from someone that has actually built the referenced design. Thanks in advance.
 
#5
Got a good laugh out of this. Thanks for advice. I actually did consider getting an old Kenwood KA-3500 (just like what I bought brand new in the late 70s or early 80s). Couldn't believe the prices on eBay and elsewhere. I built a couple of full range single driver speakers using Mark Audio 10p drivers and am pretty happy, but I have a tube amp that only puts out about 3 watts and the speakers are just on the edge of efficient enough. I will check out the ESP P3A. I also found a Class A on diyprojects.com that puts out 30 watts and is really simple.
 
#6
Did you try the article AFTER the initial Borbely amp where the authour recommends 3 or 4 additions to fix the issues? It was not done by Borbely himself but someone else. Also in this same article, the author recommends fabricating a 317/337 regulator to feed the front end because Borbely's design was weak in this area? This refers to the Borbely 60.
 
#7
I was in school and working at the time I built the amp, so I made the decision it was more important to study than troubleshoot the amp. I lost touch with the friend that bought it, so I will never know if the problem was fixed. We all went our separate ways. I retired a couple of years ago and am now getting back to the hobby I love ( and sometimes hate when things don't go as planned).
 
#9
There are plenty of good amps here on Diya that would fit your bill.

Here are a some that I have personally built:

Retro 50W (Apex AX-6):
https://www.diyaudio.com/community/threads/retro-amp-50w-single-supply.236256/
Very Simple Quasi Complimentary:
https://www.diyaudio.com/community/...imentary-mosfet-amplifier.255427/post-4844098
I have the AX-14 built but not yet tested (100W).
https://www.diyaudio.com/community/threads/100w-ultimate-fidelity-amplifier.164093/
The Apex thread has lots of designs including power supplies and preamps.

For something likely to 'work first time', Tomchr has some excellent chip amp kits at https://neurochrome.com/ and is quite responsive here on Diyaudio.

Happy building!
 
#10
I built an Apex AX6. I got 70 W 2 channels for 5 seconds out of it on 8 ohms SP2-XT speakers. https://www.diyaudio.com/forums/solid-state/236256-retro-amp-50w-single-supply-42.html
I modified the input for >100 kohms input impedance to be compatible with my PAS2 preamp. I had MPS8099 TO92 transistors with a gain of about 300.
I built it point to point since I can't make boards. NemaCE laminate 1/16" from mcmaster.com made in US. The advantage of a speaker cap amp, if a wire pops loose it doesn't produce DC on speaker & burn it. The sound is great. At 1 watt where I listen most of the time sounds the same as my .03% HD Peavey CS800s. But no fan on ST120. 72 v 6.75 a dynaco transformer regulated to 70 v. Single winding, which is what I had. Single 3300 uf rail cap, 2 3300 uf speaker caps. My 3.5" tall board fits in the ST120 case the transformer came in. I upgraded the heat sink since ST120 always had a tendency to overheat & short the output transistors. I bought it burned up. I used NTE60 output transistors which had the same SOA spec as MJ15003. I used MJ15028/29 drivers and a GE D44R4 20 mhz Ft VAS. First try fairchild TIP31c/32c drivers had poor highs. I used heat sinks on the TO220 transistors. Original ST120 drivers were 2n5320/5322 which were 50 mhz drivers. I used the original ST120 design with output transistors mounted on the heatsink & 4" long wires from driver board to O.T. I put an 8 turn coil on the driver board end of the feedback wire to inhibit oscillation. Salvage from a PCAT switcher supply. The ST120 has wire coils around the output caps to stop AM radio from the speaker wires, and a 1kohmseries1nf zobel mounted on the back of the speaker jacks. It doesn't thump at turn on or turn off.
I built an LM1875 chip amp and couldn't get it to stop motorboating. Both channels did it. Layout incompatible with point to point build. The boards from ***** were all the split supply version, whereas I built the single supply speaker cap version.
 
Last edited:
#14
Nico Ras said:
What have you available in your box of goodies, i.e. transformer, power devices, rectifier, reservoir caps, etc? I will look through my designs that may fit the components you already have.
Click to expand...
Thanks Nico Ras- I am on a cruise ship and won't be home for a couple of weeks, but that is a great idea. I do have lots of goodies. When I get back I will follow up if you are still available.
 
#15
For someone with little experience, repair is a way to start.
And can be satisfying.

Also, bear in mind that all sorts of strange component shortages exist.
It gets frustrating if you cannot get the right trivial part.
Had somebody here , posting from Sweden, wanted heat sinks from Australia...they responded late, he had an anxiety moment.
 
#17
NareshBrd said:
For someone with little experience, repair is a way to start.
And can be satisfying.

Also, bear in mind that all sorts of strange component shortages exist.
It gets frustrating if you cannot get the right trivial part.
Had somebody here , posting from Sweden, wanted heat sinks from Australia...they responded late, he had an anxiety moment.
Click to expand...
I thought about an LM3886 chip amp build then saw Mouser and Digikey have 0 available and they are backordered for at least 4 or 5 months if not more.
 
#18
indianajo said:
I built an Apex AX6. I got 70 W 2 channels for 5 seconds out of it on 8 ohms SP2-XT speakers. https://www.diyaudio.com/forums/solid-state/236256-retro-amp-50w-single-supply-42.html
I modified the input for >100 kohms input impedance to be compatible with my PAS2 preamp. I had MPS8099 TO92 transistors with a gain of about 300.
I built it point to point since I can't make boards. NemaCE laminate 1/16" from mcmaster.com made in US. The advantage of a speaker cap amp, if a wire pops loose it doesn't produce DC on speaker & burn it. The sound is great. At 1 watt where I listen most of the time sounds the same as my .03% HD Peavey CS800s. But no fan on ST120. 72 v 6.75 a dynaco transformer regulated to 70 v. Single winding, which is what I had. Single 3300 uf rail cap, 2 3300 uf speaker caps. My 3.5" tall board fits in the ST120 case the transformer came in. I upgraded the heat sink since ST120 always had a tendency to overheat & short the output transistors. I bought it burned up. I used NTE60 output transistors which had the same SOA spec as MJ15003. I used MJ15028/29 drivers and a GE D44R4 20 mhz Ft VAS. First try fairchild TIP31c/32c drivers had poor highs. I used heat sinks on the TO220 transistors. Original ST120 drivers were 2n5320/5322 which were 50 mhz drivers. I used the original ST120 design with output transistors mounted on the heatsink & 4" long wires from driver board to O.T. I put an 8 turn coil on the driver board end of the feedback wire to inhibit oscillation. Salvage from a PCAT switcher supply. The ST120 has wire coils around the output caps to stop AM radio from the speaker wires, and a 1kohmseries1nf zobel mounted on the back of the speaker jacks. It doesn't thump at turn on or turn off.
I built an LM1875 chip amp and couldn't get it to stop motorboating. Both channels did it. Layout incompatible with point to point build. The boards from ***** were all the split supply version, whereas I built the single supply speaker cap version.
Click to expand...

I have not read the whole post, just last few sentences stunt me. I built literary dozens of lm1875 amps, with boards, without boards, inverted, non-inverted, current source, you name it, i tried it all, well i need to try nested, but never experienced motorboating.
 
#19
LM1875 are available, and there are sellers in India offering ready assembled mono amps for about $1.35, using Chinese chips.
The chips from CRC, which are generic clones, are well regarded here.
We do get ready populated boards for 4440 (12V), 2030 (19V), 1875, 2050, even 3886 (most likely fake). Prices start about $3 for a two channel kit, 2.1 / 4.1 / 5.1 kits are also sold here.
Mosfet kits are also available for higher power.
Add a transformer, FM module, hardware like pots and housing, and you are ready to go.
If you are fussy, you have to search for parts.

Like I said, think about repairing first, then a ready kit, the next step will be assembling a set of parts from the same source.
Last is PCB from one place, transistors from another, pots from a third and so on...
 
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