Dead Dynaco Stereo 400 - Opinions

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I had a dead Dynaco Stereo 400 power amp fall into my lap today.
This amp did years of service as a mobile DJ amp, and then sat on a shelf for years.
A quick probing revealed s/c output transistors and maybe drivers too.
Questions - What are good replacement outputs and drivers for this amp (MJ15003/4, MJ15024/25 etc) ?.
Do the carbon composition resistors give much trouble ?.

What are these amps like when running factory new by todays standards (good, bad or ugly ?), and any improvements worthwhile doing ?.

Thanks, Eric.
Even 25 years later these units sound fairly decent.
Just repaired one i blew the crap out of a few months ago.

Even if only one output transistor is dead you should replace
all of them. While you are at it, replace all the electrolytics
on the driver boards.

All of the transistors are still available...
James Bongiorno, the designer of this amp, suggests using 2SB595/2SD525 as drivers and MJ21193/MJ21194 for outputs with MJ15024/MJ15025 as alternate outputs.

Or you can re-drill the heatsink to be a 416, and double the number of outputs. The 416 drives 2 ohms. The ultimate upgrade would be to drill the sink for a 416 and use Leach boards.

The original 2N5631/2N6031 outputs had to be series connected because they only had 50W SOA at 100V. Because the MJ21193/MJ21194 have 200W SOA at 100V they can be parallel connected.
New Millenium Upgrade ?

Hi djk, thanks for the good info.
I have not seriously studied this machine beyond a quick diagnosis session whilst the customer was watching , and I figured that for circa 1975... the output transistors are not as tough as some modern examples.
Devices like "2SB595/2SD525 as drivers and MJ15024/MJ15025 as alternate outputs." - these are easy and economical for me to get, so that route sounds good to me.
2 ohm output capability is also a must in my books, and thanks for the pointer to the 416 which I did not know about.
Fitting Leach boards is probably more of a round tuit project, but tempting never the less given the very suitable chassis, transformer, heatsink and layout of this old school engineering school mass production example.
This can be a quite serious horsepower amp as standard or modified.
I am the ultimate subjectiveist at heart, but a repairs technician/realist during the day time, so the cost/sound relationship is often skewed in the cost/reliability direction.
Please tell me if I'm wrong, but a renovate to SMPS low esr signal/bypass caps, and new output and driver transistors ought to bring it up to something reasonably decent, for LF at least.


IME physically heavy amps just sound better.
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Re: New Millenium Upgrade ?

I was never particularily impressed with this amp. It was one of the best big amps of the day (which is why we sold many - i even built some of the kits), but didn't sound anywhere as good as smaller amps of the day. PS Audio, NAIMs, Amber, Adire... i even preferred the Quad 303.

So when you fix it keep in mind mods. Allen Wright reports that during his early days in Oz to having made a living tweaking such as these with things like CCS...

diyAudio Editor
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I''ve got a Hafler 220 with one channel out.
I suspect that the repair bill will exceed $75

With the price of heatsinks and cases i'm beginning to think I would be better off gutting it and putting in 6 inverting gain clones instead. Hmmmmm WAIT!! I'll check the power supply output If it works for the clones, that amp is doomed!!

Dave, you think the clones would sound better?
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Variac said:
With the price of heatsinks and cases i'm beginning to think I would be better.

Dave, you think the clones would sound better?

I have 2 DH200s, and am part way towards making 2 100W monoblocs out of them by removing a channel from each -- one of the commercial mods was to swap in a trafo with 2x the VA rating.

IIRC the rails are a little high for a gain klone -- at least using the usual suspects.

It is a tough call on this one. Do you have a use for a single 100W channel?

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I guess I should measure what the power supply is putting out.
I assume way too high. If you know what yours puts out, I'll look over my links to various transformers online to see if there is a cheap one that would power two channels with a much higher VA rating- maybe you could keep both and just get a new trannie

I guss you'lls ay your your monoblocks are free, so why pay for a new transformer. If you go the monoblock route,maybe you should use only 2 of the transistors on each heatsink, connected to one board, to spread the cooling.

If I get the amp fixed, I have 110w. ch , to use for the active bass drivers. The sound, to my recollection is smooth, but lacking in detail- probably I should wait and try the working channel on my new speakers if I ever finish them.The six channel idea was for a separate HT system, of course that is a lower priority!!

So I guess you are saying the Haflers are a step above the Dynaco 400s in sound? If that is the case maybe it is worth fixing.

Wright Of Reply

I emailed Allen and got this reply promptly.

>I heard a rumour on a forum that you have modified/improved these in the past<
Yes, about 20 years ago I was right in the whole modding amps business - but the details are LOOOOOOOOOOOOOG gone from the memorybox. Sorry.
But basically I used to reduce the slow down caps until the amp was still stable but very much faster - and ALWAYS added a current sourse to the input differential pair (s).
I remember it to be a great sounding monster when done this way.

Now I'm into upgrading SACD players:
And building rather extreme tube preamps, which are also available as kits:

>Also are MJ15024/25 outputs transistors suitable<
No idea but they are pretty beefy - and the ones in Dyna's were really weak...
Just be really careful that any new TR doesn't make it oscillate at high frequencies - don't attempt such a change without a scope on the output when firing up - and ONLY ever fire up such a moster on a Variac!

Allen (VSEI)

Any further advice from anybody ?.
Suitable CCS schematic ?.

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Re: Wright Of Reply

One of Allen's... may be suitable



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