I have cloned a Dynaco ST80. The only circuit changes are 6800uf filter cap and output caps. My issue is that there is 3v of speaker cone deflection on start up and I can’t shake it. 3v DC measured across the outputs for a split second. Have added a 100nf x cap to the live and neutral. This actually helped a little bit as the deflection is now silent. Any ideas? I have an original ST80 and it behaves the same way. Would still like to fix it. Would an NTC thermistor help?
That is how these are.
A very early RCA designed quasi complimentary output 70W amplifier.
We used to incorporate a speaker delay relay, for that same reason.
The cause is capacitors charging up and the reactance of the gain stage along with the boot strap capacitor; C3 and coupling capacitors settling down.
A very early RCA designed quasi complimentary output 70W amplifier.
We used to incorporate a speaker delay relay, for that same reason.
The cause is capacitors charging up and the reactance of the gain stage along with the boot strap capacitor; C3 and coupling capacitors settling down.
Most of those amps didn’t use a speaker “delay”, because it’s primary function is really to protect the speaker if the amp fires up with the output fully stuck to the rail. Older cap coupled amps don’t need that function, and 3V worth of deflection isn’t enough to worry anyone if they know it won’t get worse than that - even if the amplifier fails.
People often complain about relays not being able to protect if the amp suddenly heads for the rail in operation. If you want silent turn on go ahead and use a mechanical relay. The cap will protect in the event of catastrophe. If you do fit new output caps, use one rated at the full operating voltage, or at least close to it, not just over half as most manufacturers would. Then if it needs to be used for protection it’s not being stressed unusually.
People often complain about relays not being able to protect if the amp suddenly heads for the rail in operation. If you want silent turn on go ahead and use a mechanical relay. The cap will protect in the event of catastrophe. If you do fit new output caps, use one rated at the full operating voltage, or at least close to it, not just over half as most manufacturers would. Then if it needs to be used for protection it’s not being stressed unusually.
Thank you. It’s an interesting design hence my wish to clone it with as much fidelity to the original as possible. The hard part has been trying to source transformers at reasonable prices. There are three + voltages. The original runs at 74, 72 and 36.5. My clone runs at 76, 74 and 37.5. All of the large caps are 100v. I’m heading down the rabbit warren of subjectiveism which isn’t my usual take.
Reduce the mains cap and the speaker caps to 3300 uf. That is what my ST120 has and it does not have turn on thump. I have got 73 w/ch (24.5 vac on 8 ohms speaker) both sides for 5 seconds out of it, so just enough cap does not limit the power. I do have regulated main rail, 69 v from 72 v open circuit. Now series darlington pass regulators instead of the dingbat dynaco PC14 that was collapsing voltage at 2 amps without the low gain homotaxial pass transistor that burned out before I owned the unit.
BTW the new output caps are much smaller than the dynaco ones. I left the output coils around them laying on the bottom, and it does not oscillate or pick up radio from the speaker cables. New caps are glued to the bottom. I do have 1000 ohm series .1 uf across the output jacks per greg dunn version of "TIP mod".
BTW the new output caps are much smaller than the dynaco ones. I left the output coils around them laying on the bottom, and it does not oscillate or pick up radio from the speaker cables. New caps are glued to the bottom. I do have 1000 ohm series .1 uf across the output jacks per greg dunn version of "TIP mod".
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