RS 40-987 In Wall Traf. Volume Control as preamp attenuator

[ultrachrome]

I saw a RadioShack In-Wall transformer based attenuator used as a passive preamp at the NWWA DIY Audio Meet this weekend. Apparently there was a thread on diyAudio about it but I can't find it.

Any opinions or caveats about using this as a passive pre or attenuator for a preamp?

I really quickly wired it up as a pre volume control for my Bottlehead Foreplay and it seems to work but I haven't had a chance to do any critical listening.

I'm thinking of setting my dual mono volume controls at my max listening level and using this attenuator to control the volume. Even with only 9 steps (+ mute) there seem to be enough volume steps for my needs.

 

[planet10]

I saw a RadioShack In-Wall transformer based attenuator used as a passive preamp at the NWWA DIY Audio Meet this weekend. Apparently there was a thread on diyAudio about it but I can't find it.

I hear Terry has finally gotten his in a box.... when i heard it a couple of years ago i was quite impressed with it... enuff to go out and get one for a project of my own. I got a more expensive 12-position Niles unit. I have a 3 position selector switch & chassis to go along with it... just need to get around to drilling holes for the RCAs, and wire it togther.

dave

 

[planet10]

PS... the reason you can't find the thread is because i think it was over on the Full Range Forum. There was also a bit on commentaty of it in the thread on the 1st Vancouver DIY-fest that RAW hosted.

dave

 

[ultrachrome]

I checked Full Range forum but couldn't find anything. I thought the thread was at diyAudio because the printout I saw had your avatar on it.

I'm wondering what I might want know about TVCs before I start using it.

 

[ultrachrome]

...like where should I use it with a line stage? Before or after?

I've been googling TVCs and have not come up with much except for S&B's web page but much of it seemed over my head.

 

[planet10]

...like where should I use it with a line stage? Before or after?

Usually the TVC replaces the line-stage. or you use it as the line-stages volume control.

dave

 

[spacemen12]

Hi,
Does anyone effectively try it. How does it sound? I'm looking for a passive preamp to use with my gainclone (non inverting).

Is there any concern with the fact that this attenuator was built for 8 ohm load (speakers) and we use it somewhere esle with potential different load (between cd and power amp for example).

Thnaks

 

[zBuff]

That's good point you raise there actually. You might actually gain more improvements by having no volume control device between your preamp and your poweramp, and just using the TVC for the purpose it was designed, between ampiflier and speaker.

I can't say I've tried a TVC myself yet, but I have been interested in trying them out.

 

[moamps]

The RadioShack in-wall transformer-based attenuator is intended for speaker power attenuation. It is generally not suitable for line-level attenuation for 2 main reasons. Firstly, attenuation steps are not in the appropriate dB voltage range (the attenuator is designed for power-step attenuation, not voltage). Secondly and more importantly, primary inductance is not sufficient to supply the attenuator directly from the line source, causing a dramatic rise in f3 and low-frequency distortion.

In my experience with TVCs, primary inductance should be above at least 30-40 H to avoid low-frequency rolloff (depending on the output impedance of the source). I have currently 2 TVCs published on my personal website, one of which is the autotransformer type (in the picture below).

Regards,
Milan