Advice for powering Thiel CS-3.5 speakers with Dynaco

VintEDge: Posts: 3
Join date: 2010-12-21

Post n°1

Advice for powering Thiel CS-3.5 speakers with Dynaco

by VintEDge on Fri Jan 21, 2011 4:03 pm

Hi - new guy here and I have questions- I have Thiel CS-3.5 speakers and I am powering them with about a 1964 vintage ST-70, a Pas3, and a Luxman tube output Cd player 105u. It sounds great but needs more volume- can't even turn it up to half. I wanted to use both my St-70s in the mono mode to get more power, but St-70 is a pentode, one is triode.
Questions:
Can I mono the STs even if one is a triode type?
Should I sell the triode and get another matching pentode St-70?
Sell both St-70s and buy an ST-120? or a 125 monos?
Ed

GP49

GP49: Posts: 284
Join date: 2009-04-30

Post n°2

Re: Advice for powering Thiel CS-3.5 speakers with Dynaco

by GP49 on Fri Jan 21, 2011 4:34 pm

You should not mono the ST-70s at all using the STEREO/MONO switch. The Dynaco method of running the two channels in parallel to get a 70 watt amplfier simply does not work. It combines the error-correcting negative feedback from both channels, so that it doesn't really error-correct anything; and this also wreaks havoc with the output impedance of the paralleled-channel amplifier. The only RIGHT way to use an ST-70 as a mono amplfiier is to invert the signal to one channel and take the output across the hot output connections: which is "bridging" the amplifier. This actually QUADRUPLES the original (nominal) 35 watts per channel to 140 watts (nominal) per bridged amplifier.

However, the phase inversion circuitry obviously must be as good as the rest of the amplfier, and you can no longer operate two such bridged amplifiers into any common ground switching devices such as speaker selectors or headphones.

Bob Latino

Bob Latino: Admin; Posts: 1197
Join date: 2008-11-26
Location: Massachusetts

Post n°3

Re: Advice for powering Thiel CS-3.5 speakers with Dynaco

by Bob Latino on Sat Jan 22, 2011 6:39 am

Hi Ed,

Thiel recommends a minimum of 40 WPC for the CS-3.5. The Dynaco VTA ST-120 is 60 WPC and would work well with your Thiel's in most average sized rooms. In a larger room and/or for more headroom the VTA M-125 monoblocks are probably a better choice to drive your Thiel's.

ST-70's, as GP49 has mentioned above, do not sound good in mono mode and are not worth the effort to rewire them as mono amps.

Bob

VintEDge

VintEDge: Posts: 3
Join date: 2010-12-21

Post n°4

Re: Advice for powering Thiel CS-3.5 speakers with Dynaco

by VintEDge on Sat Jan 22, 2011 6:26 pm

Then could I "bridge" both the pentode and triode St-70s to run the Thiels? Or would it be too much difference in the Sts?
I just picked up an old St-70 last night for $100 -couldn't pass that up- the guy says it worked but hasn't been run in years . Bring it up slow on a variac to see if it works? Can you give me an idea what to do?
I am seriously thinking about the ST-120 or 125s.
thanks-Ed

Bob Latino

Bob Latino: Admin; Posts: 1197
Join date: 2008-11-26
Location: Massachusetts

Post n°5

Re: Advice for powering Thiel CS-3.5 speakers with Dynaco

by Bob Latino on Sat Jan 22, 2011 6:50 pm

Bridging the amps means running them in mono and will not result in the best sound on an ST-70 especially since you have one amp in triode and one in pentode.

ONE VTA ST-120 sounds stronger than TWO stock ST-70's biamped. Although rated at 60 WPC, the ST-120 will put out 70 to 75 WPC over most of the audio band. Stock ST-70's with the original driver board only push about 25 watts per channel anyhow. Your Thiel's need a minimum of 40 WPC. IMHO you would be better served with ONE ST-120 than two original stock ST-70's.

If your ST-70's are in good cosmetic condition and both play well, you could sell both outright on Ebay and probably recoup enough money for an ST-120 kit. A new amp with all new parts will also rank higher in reliability than a 40 to 50 year old original Dynaco amp.

Bob

VintEDge

VintEDge: Posts: 3
Join date: 2010-12-21

Post n°6

Re: Advice for powering Thiel CS-3.5 speakers with Dynaco

by VintEDge on Sat Jan 22, 2011 7:18 pm

Bob,
Does the VTA ST-120 have the sound of the ST-70? If so How do I buy all wired and done ready to go?
Ed

Bob Latino

Bob Latino: Admin; Posts: 1197
Join date: 2008-11-26
Location: Massachusetts

Post n°7

Re: Advice for powering Thiel CS-3.5 speakers with Dynaco

by Bob Latino on Sat Jan 22, 2011 7:51 pm

VintEDge wrote:Bob,
Does the VTA ST-120 have the sound of the ST-70? If so How do I buy all wired and done ready to go?
Ed

Hi Ed,

The ST-120 has the same general sound as the ST-70 with lower distortion, wider bandwidth and noticeably more power. I will send you a PM through the Forum's messaging system about options on the ST-120 wired amp.

Bob