Increasing/creeping DC voltage Dynaco ST120

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Hi Guys,

I'm using two Dynaco ST120 in a tri amping setup together with two mono blocks that run the bass section of my 4 way speaker. Everything is controlled by two miniDSP in order to get the crossover paths right.
Until yesterday I used this setup as a two way/bi amping setup since I had only one miniDSP but I could now change over to a tri amping setup and I see that the mid/tweeter section which shares an output on the ST120 has an ever increasing DC voltage on the output when the speakers are connected.
For a better understanding the complete setup is:

12" bass driver with no passive crossover parts driven by a Universal Tiger (I know....)
6" mid-bass driver with no passive crossover parts driven by one channel of an ST120
50mm and 25mm mid/tweeter with passive crossover parts to sort out the HF from the MF driven by the other ST120 channel.


This mid/tweeter part builds in a couple of minutes a really high DC voltage over the output contacts and I figured it's not the channel of the amp since it starts doing that on any of my three ST120 and their channels that I have available.
I'm not sure how this can be since the amp is capacitor decoupled, I have the UpdateMyDynaco capacitors in for coupling caps and power supply caps. The boards hace the TIP31/32 updates also.

Now by looking at the schematic of the passive crossover and comparing it to a "complete" speaker with conventional three or two way passive crossover setup it seems like that it is missing a pure DC voltage dissipation path way since every speaker is coupled/decoupled via a cap and in a "normal" speaker setup there would be at least one inductor followed by the voice coil to enable charging the ST120 output caps when powered up.
When I have only a driver without any other passive parts connected like I'm doing this with the mid-bass or even before with my 12" bass driver this DC voltage does not build up. Also the voltage seems to go sometimes up and down depending if there was music played or not and I shorten the inputs to get no signal input.
Do I see this right that the much smaller caps in the DSM50/25 paths are keeping the output caps of the ST120 from fully charging to B+/2?
If so, I'm thinking about getting a 100mH inductor and maybe a 10 or higher ohm resistor to to get a small amount of DC voltage dissipation through the coupling caps and enable the overall channel to get the bias right over the coupling cap. I hope that the large inductor and relative large resistance would not over load the channel. I doubt that anything would happen to the frequency curve since this channel gets only frequencies above 1.5kHz.

Any idea?


Joined 2007
Paid Member
Hi Axel. I'm only partly following you on this, and how you have it configured but I suspect your answer is that a capacitor, although it blocks current flow, will still show a voltage on an unterminated terminal.

Take a 9 volt battery and a 1000uf cap. Connect the plus end of the cap to the plus end of the battery. Now measure the voltage from battery negative to the floating end of the cap. It will show a voltage.

I don't know if that helps or not.
Well, as a test I let the mid/tweeter channel charge up, took off the speaker and connected the mid bass driver and the cone got a pretty nice deflection and made a popping noise. The cone returned very quickly back to neutral and the voltage was 0 again and stayed 0 for some time.
I will check today what the voltage over the coupling cap is when this happens and compare it to the mid bass channel.
I remember, when I rebuilt the amps the voltage drop over the coupling caps with a "normal" speaker setup connected was roughly half of B+ what seemed to be right according to a drawing with measured voltages from the net.
What you're seeing is perfectly normal.

Stereo 120 is a capacitor coupled amp. Depending on the vintage, there is no DC path to ground unless provided by the speaker. Thus, driving your mid-range/tweet, which owing to the coupling caps, look like an open circuit at DC, the DC output voltage of the 120 will creep upward.

The solution? Just put 470 Ohm 1W resistor across each pair of binding posts.
That will largely keep the DC at the output gone. There may be a little owing to leakage current in the output cap, but it won't be significant.

Hope this helps.

Update My Dynaco
Thanks guys,

I think I will go with the recommended 470 ohm resistor over the binding posts.
Would it be helpful to also a coil inductor in series with the resistor in order to avoid getting any AC path going? I have a 100mH inductor that I could put in front of the R?
Am I to understand that this /these are oem St120s'.. Really !?
Repopulate the contraptions using the updatemydynaco innards (good stuff, in my experiences)
OR throw the stock 120's in the nearest Dumpster.
Yesss ! they Do sound That bad. Even a 20$ Ebay " YJ " Chi-Fi chip amp gizmo Seriously outclasses an ST120.. Try It? as a wee eyeopener?
Tough to soar like an Eagle when flying with Turkeys ;)
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