Radford STA25 Mark 4 - passive preamplifier for?

[opt817]

Hello,

after many years I finally have opportunity to buy Radford STA25 Mark 4, limited series of 100 pieces.
I ask respected forum members for a help, because there is very little literature about this model?

1. Input sensitivity of STA Mk.4 ? (STA mark5 should have 500mV)

2. Best preamp for it, possibly passive, since I'll connect in CD player with about 2V and phono also with about 0.5 to 1V signal?

Thank you for your valuable suggestions!

[Barry]

The input sensitivity for the Mk.III and Mk.V is 500mV. I don't see why that of the Mk.IV should be any different.

A 'passive' preamp will be fine to use with a CDP, but if you want to also play records then you will need a separate phonostage that can output at least 500mV.

How much is your budget?

[opt817]

My budget is up to 900 pounds. I already have a phono with output between 0.5 and 1V. So I need passive preamp with 3 RCA inputs and at least 1 RCA output? My CD player can use XLR too.

[Barry]

Yes that is exactly what you need - a passive pre with an input selector for 3 inputs.

£900 will buy you several likely contenders. I'll have a look and find some for your consideration.

[Lawrence001]

Music First Audio TVCs and Hattor ladder based passives are both popular. If you're lucky you might find an early MFA TVC at the top of your price range.

Sent from my HEY2-W09 using Tapatalk

[mik_rik]

Promitheus TVCs are also excellent and usually cheaper than the MFA tvc,s not as common though
I have1 and use it with my STA15

[opt817]

Which model of Promitheus TVCs are you using? Any problems with limited number of steps?

[mik_rik]

no problems at all .If I recall correctly it has a slight amount of gain
Mine has no model number ,I believe they were hand built by the company owner
It has 4 inputs and 2 outputs
I have taken some photo's of the front,back and of the inside but have had no luck posting them yet,I'll keep trying
Mick

[guy]

I use an NVA P50 SA (stepped attenuator) with my Art Audio Quintet - love it.
The only comparison i have is with the older NVA P50, Cermet attenuator. Stepped attenuator made large improvement, i was really surprised by how much more detail/“authority” it gave the system.

I understand, from other users that the P90 (separate control for each channel) is a further improvement, but the NVA P50 SA is a keeper for me - very satisfied.

https://nvahifi.co.uk/products/nva-p50sa-mkii-passive-preamplifier?srsltid=AfmBOoojcyiOFylrurBJ3d5Nk3g-W3hFiQ7hd12-JzmPGIfipNhI8bvK

EDIT: i was worried about moving to a stepped attenuator as my alarm clock/radio volume is in steps, and i can never get the level i want :steam:
No problems with the P50 SA in this regard, i guess that the resistors values at low volume levels are very carefully chosen?

[opt817]

Thank you for suggestions. So both passive attenuators are working, ones with stepped attenuators and other option is autotransformer type, like Promitheus TVC, Transformer Volume Control.
According to high price the best solution is Music First Audio TVC? For best price - Promitheus TVC?
Question - what is your experience with both types of passive attenuators with Radford STA25?

[opt817]

To have more possibility to tune sound to warmer side, I'm also interested in active preamp, mainly tube preamp for Radford STA25. It will be nice to have MM phono input also?
Any good recommendation for price up to 1500 GBP? Something from Audio Note, etc?

[The Black Adder]

I have an updated Croft that would be a perfect match with the Radford.

Happy to match your budget.

Sent from my BBC Microcomputer 32K

[opt817]

Thanks for offer, but since brexit we, mainland desert, have great difficulty to get anything from UK, because of customs. Last time I was importing some small parts from UK, custom officers asked from me to send them all my email correspondence with UK company.
Another problem with Croft is, there is no support outside UK, so I have to do a lot of paperwork for import, then if anything happens, temporary export, again import with custom duties...
Thanks again for nice offer.
Is there a chance, that UK will go back in EU?

[Barry]

Hello Ales,
Sorry for the late reply. I was going to explain the various "passive preamp" options: series resistor networks; shunt resistor networks; resistor ladder networks; tapped transformer, and tapped autoformer. All have their pros and vons. Put simply the simplicity of a single component in the signal path is attractive. The disadvantage is in having enough descrete steps in order to adjust the volume, and in the case of trans/autoformers, arranging for the wiper to be a 'make before break', yet have a resistive element in the wiper so that any transformer turns are not shorted out when the control is operated (as far as I'm aware only Telefunken provided this on their transformer attenuators, though I expect the BBC paid attention to this aspect on their devices).

But the biggest drawback is that none of these devices (unless multiple ganged) offer a constant impedance to both the source and to the load. That is why the simplest of preamps have an active network in front of the potentiometer and after it, so as to buffer the impedance change.

Like you I have a Radford valve amp (an STA15 Mk.III) and have used it with a Levinson ML10A preamp to good effect.