25 Watt 8 Ohm in Dynaco clone

[lineup]

Here is my first version
of my clone attempt of Dynaco ST-80 .... from 1969

First version I have not changed too much.
- Using BD139 / BD140 TO126 replacing some TO39
- Using the good 2N5551 for input Q1
I have kept 2N3055 for the N Channel Quasi output
-----------

It is a bit unusual amplifier.
As you can see in my attached diagram,
the only contact between 2 Transistor voltage amp and the output follower stage
... is C3 / C4 coupling and the global feedback R9 / R4 = Gain.
Gain is ~14
---------------

Voltage supply is 74 Volt DC.
Which is equal to a modern amp 2x37 Volt.
Lowest THD distortion is around 25 Watt RMS into 8 Ohm.
THD is only 0.015 %. Very good value for such an old amplifier!
Upper bandwidth is limited to ~180 kHz
--------------------------------

I will try some better modern Output Devices, like MJL21194 for example.
I will also better figure out how to set the bias in output stage
to an optimal value.

I'll be back with more!
Analyses, tests and modifications.


Regards
lineup

[taj]

Lineup,

I have one of these (not working). Everyone tells me they sound terrible and aren't worth fixing, so I've left it in my spare parts (junk) box.

Having said that, I don't want to discourage you from working on this -- especially if you determine that they do/can sound good, or that some simple (or not) modifications might create some worthwhile improvements.

If you decide to create PCB's, make them the same size as the original so they can fit in the "slots" on the chassis. And stick to a pair of TO3 devices so the same heatsink will work.

Speaking of heatsinks, those Stereo-80 heatsinks are TINY.

I'll be watching your progress.

[lineup]

Originally posted by taj
Lineup,

I have one of these (not working). Everyone tells me they sound terrible and aren't worth fixing, so I've left it in my spare parts (junk) box.

Having said that, I don't want to discourage you from working on this -- especially if you determine that they do/can sound good, or that some simple (or not) modifications might create some worthwhile improvements.

If you decide to create PCB's, make them the same size as the original so they can fit in the "slots" on the chassis. And stick to a pair of TO3 devices so the same heatsink will work.

Speaking of heatsinks, those Stereo-80 heatsinks are TINY.

I'll be watching your progress.

Many reports say the same thing.
This amplifier is not the best - at least in original standard version.

Why I am interested, is that I may learn something new from working with it.
I probably will not build it, for real.
I will try to make it work as good as possible in virtual testings.

When this is over.. I hope I know a bit more
of the of the tricks and bits that can be useful
when making amplifier designs.

So far I see not much in my simulations
that would say this one is not as good as modern typical Class AB amplifiers.

01 Watt: THD 0.050%
25 Watt: THD 0.015
Into 8 Ohm load, 25W is +/-20Volt peak
from a supply that is +/-37Volt.

thanks for your comment, taj
we will see when I get time to work with this Dynaco a bit more
Right now have some other things

Regards

[Tube_Dude]

Originally posted by lineup
[B]Here is my first version
of my clone attempt of Dynaco ST-80 ....

Hi Lineup

If you connect R11 (10k) to the left side of R21 (the 1000 uf capacitor side) the power supply rejection ratio will be much better ...

Regards

[unclejed613]

i have a complete system, st-80 amp, pat-4 preamp and fm-5 tuner all in working condition.... i think the amp needs a recap, but it still works. i don't like the clipping behavior, typical of a Q-C OTL circuit from that era.

[taj]

Originally posted by lineup


I probably will not build it, for real.
I will try to make it work as good as possible in virtual testings.

Regards

Well, if you gurus find any way to drastically improve it, using the same form factor, (same size PCB with a pair of TO-3 output devices) then the world will have a worthwhile retrofit project available to pull these out of our junks boxes and resurrect these dogs. Their cases are uniquely Dynaco and a shame to waste on substandard audio components.


I'd love to build your final design, if it looks good on paper. I'll report back on the sonics.

..Todd

[taj]

Excuse the newbie question. I'm just learning.

Where is the output taken in this schematic? Before C7 or before R30?

..Todd

[taj]

Never mind... I realize now, R30 is the load. doh!

My mental heatsink warmed up a few degrees over that puzzle.

..Todd

[lineup]

Originally posted by Tube_Dude

Hi Lineup

If you connect R11 (10k) to the left side of R21 (the 1000 uf capacitor side) the power supply rejection ratio will be much better ...

Regards

I see what you mean, Tube_Dude.
But I am not sure this is good.
The designer has built, using the two amplifiers separate:
1. Two inverting transistors = VAS, with HIGH gain
2. Power push-pull Follower

Anybody else think it is a good idea to put the RC filter before R11?


I will redraw the circuit, to better understand the BIAS Control.
I did test some different DIODES for D2/D3.
In description says these 2 diodes should give a drop of ~1.6V at 100mA.
( because of 36V/400 Ohm (R16) = 90mA, R17 also 400 Ohm is to put away those 90mA to Ground. )

I am actually planning to replace D2/D3, with a BD139 VBE multiplier
and I may also have to change value of R20
to achieve matching current in Q3 Q4.
I did some investigation of idle bias in output
and currents in Q3 / Q4, and my figures did NOT look very alright.

I am sure it comes because I have replaced Q3 / Q4 with BD139 / BD140.
The old TO39 may have had different VBE drops.

[lineup]

Originally posted by unclejed613
i have a complete system, st-80 amp, pat-4 preamp and fm-5 tuner all in working condition....

i think the amp needs a recap,
but it still works.

i don't like the clipping behavior, typical of a Q-C OTL circuit from that era.

unclejed613

I think, you put the finger at the crucial point here!

Modern amplifiers, even if they get old, have often only Electrolytics in power supply.
All other capacitors are are types that does not dry out with age.

Older Classical Amplifiers is dependent on ELECTROLYTICS IN CIRCUIT.

For good and proper operation.


I mean, in this case of Dynaco ST-80

1. Input cap, might be a solid film cap - but can also be a wet capacitor in some Amps
2. C3 C4 ---- 35 uF, is told should be non-polar
3. C5 ---- 500 uF effect the gain of VAS. If dry low gain.
4. C6 ---- 50 uF, non-polar
5. C7 ---- Output Cap ----- 4700uF BIG electrolytic!

6. ... of course also C11, 1.000uF and C9, 4.700uF
Power supply electrolytics will DisFunction if DRY OUT.


Remember
that 2007 - 1969 is a long long liong long time, for Electrolytic caps.

Almost 40 years can make the best of the best of capacitors
to be totally useless.
At least for anything in AUDIO

Regards
lineup