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Old 21st July 2013, 11:45 AM   #1
Pani is offline Pani  India
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Default Volume control after the preamp stage or before ?

Friends,
I am trying to get a preamp custom made. The preamp has a single gain stage and uses Russian 4P1L DHT tube. The output impedance is about 1250 ohms and gain is 12db. I plan to use a Dave Slagle Autoformer for the volume control. My question is, should I use the autoformer before the preamp stage or after ?

The preamp will drive a Wavac EC-300B power amp which has an input impedance of 100 kohms.

If we use the autoformer volume control before the preamp stage then two things will happen:

1. The source will see/drive the volume control directly.

2. The volume control will be attenuating/manipulating very low level signal which means more distortion.

If we use the autoformer after the preamp gain stage then the autoformer will be directly driving the power amp so I am not sure how the power amp would react to the differing output impedance of the autoformer.

Please suggest which way to go in this situation.
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Old 21st July 2013, 12:10 PM   #2
Mooly is offline Mooly  United Kingdom
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Pani, I've moved this over to the valves forum as it seems more appropriate, and you'll probably get a better response here.

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Old 21st July 2013, 01:12 PM   #3
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Becarefull with such autoformer controles. If there are any C in serie you get a resonance circuit.

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Friends,
I am trying to get a preamp custom made. The preamp has a single gain stage and uses Russian 4P1L DHT tube. The output impedance is about 1250 ohms and gain is 12db. I plan to use a Dave Slagle Autoformer for the volume control. My question is, should I use the autoformer before the preamp stage or after ?

The preamp will drive a Wavac EC-300B power amp which has an input impedance of 100 kohms.

If we use the autoformer volume control before the preamp stage then two things will happen:

1. The source will see/drive the volume control directly.

2. The volume control will be attenuating/manipulating very low level signal which means more distortion.

If we use the autoformer after the preamp gain stage then the autoformer will be directly driving the power amp so I am not sure how the power amp would react to the differing output impedance of the autoformer.

Please suggest which way to go in this situation.
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Old 21st July 2013, 01:18 PM   #4
Pani is offline Pani  India
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Quote:
Originally Posted by esltransformer View Post
Becarefull with such autoformer controles. If there are any C in serie you get a resonance circuit.
I am planning to have a transformer coupled output stage at the preamp, so there will not be any C at the output. But the question is where should the autoformer be placed in the context of the preamp ?
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Old 21st July 2013, 01:40 PM   #5
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The volume controle has to be close to a tube input so no coax cable after the controle. Also be careful how much voltage you put in the controle, overload is more a problem then a small signal on the controle.

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I am planning to have a transformer coupled output stage at the preamp, so there will not be any C at the output. But the question is where should the autoformer be placed in the context of the preamp ?
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Old 21st July 2013, 04:31 PM   #6
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The main advantage of using the autoformer volume control at the output is that it can greatly reduce that 1250 ohm 4P1L output impedance for signal attenuating (less than 0dB) volume settings - which is almost always the operating situation. In addition, tube noise at the output is reduced along with the volume setting. Which means that the preamp's signal-to-noise ratio is maintained as the volume is lowered.
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Old 21st July 2013, 08:32 PM   #7
Magz is offline Magz  United States
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I'm using the slagleformers (bent audio remote controlled version) at the output of my 26 preamp. It sounds excellent, with lots of drive. I'm using the autoformers as my output transformers in a lossy parafeed configuration, it sounds much more transparent than using an OPT before the VC, which I tried first. The 26 has a cascoded CCS on the plate and the output to the VC is taken from the mu output of the CCS. The slagleformer is stacked 3x3 for about 150H inductance.

In this case, the presence of the parafeed C before the VC is actually beneficial, and allows a corner frequency of 4.8Hz.
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Old 21st August 2013, 12:22 PM   #8
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Default LDR Passive Preamp Controller Board w/ Remote

You might want to consider an LDR based passive preamp volume control. We've had several customers try our LDR3x unit who have been using transformer based volume control and they report surprising (to them!) improvements over TVCs. A quote from a recent LDR3x customer can be found here.
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Old 21st August 2013, 12:58 PM   #9
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A transformer also has a capacitive input above certain resonant frequency of the primary winding and interwinding capacitance, and also as capacitances between all the windings between them and to ground‼
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Old 21st August 2013, 03:43 PM   #10
Merlinb is offline Merlinb  United Kingdom
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If you care more about noise, then the volume control should go after the preamp.

If you care more about headroom then it should go before the preamp. Output impedance may also be improved in this case, depending on the control setting, as Ken said.
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