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Thread: Passive amp set up for Radford ST25 amplifier

  1. #1
    Join Date: Mar 2018

    Location: York UK

    Posts: 10
    I'm David.

    Default Passive amp set up for Radford ST25 amplifier

    Dear all

    I have just inherited an home built amplifier from a friend that was built using the Radford ST25 circuit in 1986. The mains and output transformers were direct from the Radford factory, wound by their super expert lady winder who they got out of retirement in 1986 when they started building them again! I have replaced a number of capacitors that had passed their sell by date and generated a lot of smoke when I switched it on, and the amp is now back up and running after nearly 20 years of being sat in someone's bedroom cupboard. I have tried it connected directly with my level adjustable CD player and it sound pretty good through my Quad ELS 57 speakers. However, when I tried it in my system running a passive amp with a couple of Glass House mono 47 step attenuators, the sound is no where near as good, very thin and lacking in energy. I use the passive with my Leak Stereo 20 power amp with no problems at all and it sounds great with this amp. I also have to turn the attenuators well past the 80% level to get an decent volume. Could the degradation of signal be due to mismatch of impedance. I have now measured the load resistors that are connected to the the attenuators and they read 50k. From the information on the amp circuit diagram, there is a 100k resistor from the input to earth. Could this be causing a mis-match on impedence between the passive and power amp. If so, suggestions about how to address the issue would be more that welcome since I would love to try the amp out properly set up in my system.

    Kind regards

    David
    Last edited by dpgals; 02-04-2018 at 16:09.
    David
    Equipment Garrard 401 Slate Plinth SME V Van Den Hul Colibri cartridge
    Lector Phonostage
    Raspberry Pi IQ Audio Dac running Volumio distribution
    Own build high quality silver wired passive preamp in Leak Varislope pre-amp shell
    Leak Stereo 20
    Quad ELS 57 speakers (some of the last built SNs in the region of 52000)
    REL Strata V subwoofer

  2. #2
    Join Date: Jan 2009

    Location: Essex

    Posts: 18,439
    I'm openingabottleofwine.

    Default

    The sensitivity of the Radford STA25 is 500mV for full output. For the Leak Stereo 20 it is 100mV. I think that is where your problem is (a difference of 14dB), and why you have to 'turn up' the control on your passive when using the Radford.

    You say your CD has adjustable outputs. Are they set at full output when you use the Radford?
    Have you listened to this month's choice in the Album Club?

    Barry

  3. #3
    Join Date: Mar 2018

    Location: York UK

    Posts: 10
    I'm David.

    Default

    The input sensitivity of the Leak has been lowered so I think it is now similar to the Radford. The CD ( a Shandling) has a remote volume control that you can set between 0-100 so as to adjust the output. I feel it is the mismatch between the two resistors giving the problem so not certain what to do - increase or decrease either the load resistor in the passive or the resistor in the input line of the amp. Thanks for the response.

    Regards

    David
    David
    Equipment Garrard 401 Slate Plinth SME V Van Den Hul Colibri cartridge
    Lector Phonostage
    Raspberry Pi IQ Audio Dac running Volumio distribution
    Own build high quality silver wired passive preamp in Leak Varislope pre-amp shell
    Leak Stereo 20
    Quad ELS 57 speakers (some of the last built SNs in the region of 52000)
    REL Strata V subwoofer

  4. #4
    Join Date: Mar 2018

    Location: York UK

    Posts: 10
    I'm David.

    Default

    Barry

    It look like the Leak has an input impedance of 1M ohm so if I change the input resistor load on the Radford to 1M ohm then I think all will be well in terms of SQ.

    Regards

    David
    David
    Equipment Garrard 401 Slate Plinth SME V Van Den Hul Colibri cartridge
    Lector Phonostage
    Raspberry Pi IQ Audio Dac running Volumio distribution
    Own build high quality silver wired passive preamp in Leak Varislope pre-amp shell
    Leak Stereo 20
    Quad ELS 57 speakers (some of the last built SNs in the region of 52000)
    REL Strata V subwoofer

  5. #5
    Join Date: Jan 2009

    Location: Essex

    Posts: 18,439
    I'm openingabottleofwine.

    Default

    Quote Originally Posted by dpgals View Post
    Barry

    It look like the Leak has an input impedance of 1M ohm so if I change the input resistor load on the Radford to 1M ohm then I think all will be well in terms of SQ.

    Regards

    David
    Certainly the 100k input impedance of the Radford will 'load' the 50k potentiometer of your passive far more than the 1M input impedance of the Leak, and thus affect the setting for a given volume level. I would need to have a look at the circuit diagram of the Radford to see if increasing the input impedance to 1M is acceptable (it may form part of a bias resistor chain to the first valve).

    You say the sensitivity of the Leak has been reduced to match that of the Radford. How was that done?
    Have you listened to this month's choice in the Album Club?

    Barry

  6. #6
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 1,380
    I'm Andrew.

    Default

    Is there any unusual about the passive pre-amp? The vogue for having "only one resistor in the signal path" has led to some unusual designs that I personally think are quite daft. That's my opinion of course, though I'm prepared to back it up with some science.

  7. #7
    Join Date: Jan 2009

    Location: Essex

    Posts: 18,439
    I'm openingabottleofwine.

    Default

    I've just had a look at the circuit diagram for the STA25, and can see no reason why the grid resistor to the input EF86 could not be increased to 1Meg. But I'm not a valve electronics expert, so it might be best if someone else who is could confirm. Perhaps you could ask Anthony TD, who designs valve power amps and is very knowledgeable about such things.
    Have you listened to this month's choice in the Album Club?

    Barry

  8. #8
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 4,343
    I'm Alan.

    Default

    1 Meg is perfectly fine for the EF86, spec sheet says it can be up to 3 Meg.


  9. #9
    Join Date: Jan 2009

    Location: Essex

    Posts: 18,439
    I'm openingabottleofwine.

    Default

    Oops! Sorry Alan I forgot to mention that you too are one of our resident valve experts. Most remiss of me!
    Have you listened to this month's choice in the Album Club?

    Barry

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