Strange output behavior, VTA ST70 with speakers

Hello all,
I have a rebuilt St70 with the low gain VTA driver board.  Sounds so nice.  I pair this up with a set of Klipsch R-51M bookshelf speakers.  These speakers roll off below 70-80 Hz, so I use a powered sub to give me the lows using the high level inputs from the ST70 speaker terminals.
While evaluating the response of the system, I heard a strong peak at about 50 Hz and rapid drop after that.  After various testing with my dummy 8 ohm load, I found that with the dummy load, the voltage output from the ST70 was flat down to 20 Hz.  But when connected to the speaker, the voltage would drop at 60 Hz, then jump up 50% or more at 50 Hz, then fall off rapidly.  The same test with a different speaker gave different results, but were not flat below 70 Hz.
So it looks like the low frequency impedance is affecting the voltage output from the transformer, which for just the bookshelf speakers is not noticeable, but once a high level sub is connected, it sounds awful.

Has anyone else observed this behavior with their ST 70?  Any ideas on how to correct it so that I can still use the sub high level inputs?  I also measured the voltage out of the 16 ohm terminal with the speaker connected to the 8 ohm terminal, and the 16 ohm terminal showed the same troublesome response.

I hate to buy a preamp with a sub out to solve this, I like to have minimal gear.

Thanks for any advice or input.

Steve

docswanson wrote:Hello all,
I have a rebuilt St70 with the low gain VTA driver board.  Sounds so nice.  I pair this up with a set of Klipsch R-51M bookshelf speakers.  These speakers roll off below 70-80 Hz, so I use a powered sub to give me the lows using the high level inputs from the ST70 speaker terminals.
While evaluating the response of the system, I heard a strong peak at about 50 Hz and rapid drop after that.  After various testing with my dummy 8 ohm load, I found that with the dummy load, the voltage output from the ST70 was flat down to 20 Hz.  But when connected to the speaker, the voltage would drop at 60 Hz, then jump up 50% or more at 50 Hz, then fall off rapidly.  The same test with a different speaker gave different results, but were not flat below 70 Hz.
So it looks like the low frequency impedance is affecting the voltage output from the transformer, which for just the bookshelf speakers is not noticeable, but once a high level sub is connected, it sounds awful.

Has anyone else observed this behavior with their ST 70?  Any ideas on how to correct it so that I can still use the sub high level inputs?  I also measured the voltage out of the 16 ohm terminal with the speaker connected to the 8 ohm terminal, and the 16 ohm terminal showed the same troublesome response.

I hate to buy a preamp with a sub out to solve this, I like to have minimal gear.

Thanks for any advice or input.

Steve

ST70 with the low gain vta board has not so much NFB, thus it has a hign output impedance and
is affected much by speaker resonances.

Increasing NFB will solve this at the cost of lower gain.

I am talking about R7 ( 7.5k) Decreasing to 2k is a recommendation.

That makes sense, and I was afraid of that. I guess I will have to splurge an a preamp with a sub out. Looking at the Schiit Saga +.

I wouldn't recommend going below 3.9K on the NFB resistors (R7 & R8) but that's a simple cheap change.
20c worth of parts instead of $300+

Running a low gain VTA in my ST70 and KT66 tubes. My bookshelfs are a tad larger being the Fritz Carrera's. No need to have a sub with these, they go down to 35Hz. I also run through a Parasound Halo P5 since I have a 4 track RTR and TT. Highly recommend both speakers and pre. No need to a sub but if you need one I have a Velodyne Optimum 10 and a Sunfire super junior I would be willing to part with. Both in excellent condition.

I'm surprised that connecting a powered sub would have that effect, even with a high impedance output. The high level input circuitry of a powered sub is designed to help avoid this problem specifically. Am I missing something?

ALL speakers have a varying impedance, and most have a high peak in the 40-70Hz range (more like 50-60 ohms), so what you are experiencing is normal.