Quad 909 pre-amp matching

[bevwil]

Currently have a  Quad 909/99 combo for many years but getting an itch to try a valve pre amp. But have been warned about impedence inputs/outputs, which i dont understand having no technical knowledge whatsoever. Thinking of a Vincent  SA T7. Would this be a disaster? Or can anyone recommend other pre-amps.  Would also like to hear from Vincent owners out there . 

Edited March 18, 2020 by bevwil
Needed to tidy up explanation

[Bronal]

OK, looks like no one wants to have a shot at this so I will.

 

From my experience, the four factors you need to consider are (A) the gain of the preamp; (B) the input sensitivity of the power amp; (C) the output impedance of the preamp; and (D) the input impedance of the power amp. A/ B and C/D go together.   A preamp with a high gain can overload a power amp with a high input sensitivity.  If the output impedance of the preamp is too high relative to the input impedance of the power amp, then overall performance can be compromised, typically producing woolly bass. 

 

In the 'old days' input impedances of power amps were generally 47 kOhms for SS amps and 100 kOhms for valve amps.  However input impedances for SS amps have gone down considerably in recent times.  The input impedance for the Quad 909 is 20 kOhms, which is common for a modern SS amp.  The rule of thumb for good bass used to be 1:100 preamp output impedance to power amp input impedance, but I have seen varying ratios quoted, down to as low as 1:10.  I wouldn't go below around 1:50 myself.  I can't find the output impedance for the STA-7, but I would hazard a guess at it being around 500 to 600 Ohms (a common figure for valve preamps).  This means it could be marginal for use with the 909 in terms of the quality of bass reproduction.

 

The 909 has a quoted input sensitivity of 775 mV for full output, which is medium to high.  However the gain of the STA-7 is 13.3 dB, which is also medium to low by current standards.  This means the STA-7 should drive the 909 well, and any excess gain can easily be tamed by the use of attenuators on the power amp inputs, such as those made by Rothwell.

 

This doesn't mean that the STA-7 won't work with the 909 to your satisfaction, but obviously an audition would be desirable before you decide.

 

Edited March 20, 2020 by Bronal

[bevwil]

Thanks Bronal. Much appreciated.  I'll keep you posted.

20 hours ago, Bronal said:

OK, looks like no one wants to have a shot at this so I will.

 

From my experience, the four factors you need to consider are (A) the gain of the preamp; (B) the input sensitivity of the power amp; (C) the output impedance of the preamp; and (D) the input impedance of the power amp. A/ B and C/D go together.   A preamp with a high gain can overload a power amp with a high input sensitivity.  If the output impedance of the preamp is too high relative to the input impedance of the power amp, then overall performance can be compromised, typically producing woolly bass. 

 

In the 'old days' input impedances of power amps were generally 47 kOhms for SS amps and 100 kOhms for valve amps.  However input impedances for SS amps have gone down considerably in recent times.  The input impedance for the Quad 909 is 20 kOhms, which is common for a modern SS amp.  The rule of thumb for good bass used to be 1:100 preamp output impedance to power amp input impedance, but I have seen varying ratios quoted, down to as low as 1:10.  I wouldn't go below around 1:50 myself.  I can't find the output impedance for the STA-7, but I would hazard a guess at it being around 500 to 600 Ohms (a common figure for valve preamps).  This means it could be marginal for use with the 909 in terms of the quality of bass reproduction.

 

The 909 has a quoted input sensitivity of 775 mV for full output, which is medium to high.  However the gain of the STA-7 is 13.3 dB, which is also medium to low by current standards.  This means the STA-7 should drive the 909 well, and any excess gain can easily be tamed by the use of attenuators on the power amp inputs, such as those made by Rothwell.

 

This doesn't mean that the STA-7 won't work with the 909 to your satisfaction, but obviously an audition would be desirable before you decide.

 

 

[stereo coffee]

On 19/03/2020 at 12:21 AM, bevwil said:

Currently have a  Quad 909/99 combo for many years but getting an itch to try a valve pre amp. But have been warned about impedence inputs/outputs, which i dont understand having no technical knowledge whatsoever. Thinking of a Vincent  SA T7. Would this be a disaster? Or can anyone recommend other pre-amps.  Would also like to hear from Vincent owners out there . 

You should consider passive attenuation with the 775mv sensitivity figure. Less is more applies 

[bevwil]

THanks Bronal and Stereo Coffee. I went ahead and bought one. ( Vincent Sa T7). Not a total disaster,   but  as you both warned , far too much gain. Volume knob rarely gets above  8pm and that,  coupled with their hopless   remote  volume,  has made for a frustrating time so far. Sound quality is promising however, with moments of "magic" ....but  most of my  listening experience  has been fine tuning a very sensitve volume control to find an ideal balance  between inaudibilty and being simply shouted at.  Rothwell attenuators look like the go. 

[bevwil]

Just now, bevwil said:

THanks Bronal and Stereo Coffee. I went ahead and bought one. ( Vincent Sa T7). Not a total disaster,   but  as you both warned , far too much gain. Volume knob rarely gets above  8pm and that,  coupled with their hopless   remote  volume,  has made for a frustrating time so far. Sound quality is promising however, with moments of "magic" ....but  most of my  listening experience  has been fine tuning a very sensitve volume control to find an ideal balance  between inaudibilty and being simply shouted at.  Rothwell attenuators look like the go. 

 

[thejt]

On 19/03/2020 at 8:41 PM, Bronal said:

OK, looks like no one wants to have a shot at this so I will.

 

From my experience, the four factors you need to consider are (A) the gain of the preamp; (B) the input sensitivity of the power amp; (C) the output impedance of the preamp; and (D) the input impedance of the power amp. A/ B and C/D go together.   A preamp with a high gain can overload a power amp with a high input sensitivity.  If the output impedance of the preamp is too high relative to the input impedance of the power amp, then overall performance can be compromised, typically producing woolly bass. 

 

In the 'old days' input impedances of power amps were generally 47 kOhms for SS amps and 100 kOhms for valve amps.  However input impedances for SS amps have gone down considerably in recent times.  The input impedance for the Quad 909 is 20 kOhms, which is common for a modern SS amp.  The rule of thumb for good bass used to be 1:100 preamp output impedance to power amp input impedance, but I have seen varying ratios quoted, down to as low as 1:10.  I wouldn't go below around 1:50 myself.  I can't find the output impedance for the STA-7, but I would hazard a guess at it being around 500 to 600 Ohms (a common figure for valve preamps).  This means it could be marginal for use with the 909 in terms of the quality of bass reproduction.

 

The 909 has a quoted input sensitivity of 775 mV for full output, which is medium to high.  However the gain of the STA-7 is 13.3 dB, which is also medium to low by current standards.  This means the STA-7 should drive the 909 well, and any excess gain can easily be tamed by the use of attenuators on the power amp inputs, such as those made by Rothwell.

 

This doesn't mean that the STA-7 won't work with the 909 to your satisfaction, but obviously an audition would be desirable before you decide.

 

THis is a fairly scientific approach, I've never really considered it in this way. Thanks alot, much to consider.