Slagle Autoformer

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JohnG
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Slagle Autoformer

Post by JohnG » Thu Mar 29, 2018 10:06 am

I,ve taken the plunge and I think I have purchased a Slagle Dual Mono Autoformer Module, that has 1.25db Steps.
It is fully assembled in a case and has the additional option of being used as a Headphone Preamp.
It will hopefully be with me in 10 Days or thereabouts.
Would any one intending on visiting Owston on the Saturday, like to have a listen to it.
I have been assured by the vendor, that a Senny HD650, works very well with it giving a very detailed presentation.
I also am awaiting the arrival of a DACT, that was purchased for me in a Group Buy.
So if either arrives as planned, I can at some time do a A/B Comparison and work out how passives pre's work in my evolving system.

JohnG
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Re: Slagle Autoformer

Post by JohnG » Sat Mar 31, 2018 10:29 am

Seller of the AVC seems well versed in the need to match the AVC, to a compatible Output Power of a device coupled to it.
I am looking at a DAC with a RCA Output of 2Vrms and I have seen the XLR Connection can be higher than this.
I was also sent a few links by the seller to see a few not so common brand DAC's that should be a match.
This is a whole new field of investigation for me, so I am treading carefully.
Any thoughts aired on this will be well received.
The Paypal invoice for the AVC is yet to arrive, so the sale is not secure at present.

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IslandPink
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Re: Slagle Autoformer

Post by IslandPink » Sat Mar 31, 2018 11:42 am

It's not just voltage you need, John - it's a decent output impedance and current in the DAC output stage . What has Dave suggested ?
Or was it not Dave selling, but a previous owner ?
Drive it right and it should be superb.
White flags of winter chimneys waving truce against the moon ( Mitchell )

JohnG
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Re: Slagle Autoformer

Post by JohnG » Sat Mar 31, 2018 12:18 pm

It's no secret I am a numpty when it comes to discussing electronics, capable of changing a remotes batteries and the like.
I would like to get the best from this device.
So how does this device get "driven right".

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IslandPink
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Re: Slagle Autoformer

Post by IslandPink » Sat Mar 31, 2018 1:10 pm

Well I'd take a stab and say 2k-ohms or less output impedance and 20ma ( if valve ) current in what's driving the TVC. Dave should be able to pinpoint it more exactly.
White flags of winter chimneys waving truce against the moon ( Mitchell )

JohnG
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Re: Slagle Autoformer

Post by JohnG » Mon Apr 02, 2018 7:21 am

Following is the reply I received from Dave Slagle about my setting up a AVC as attenuator for a Headphone, I have included the web link he supplied, as this may be of further interest to anyone wishing to learn more about Headphones design and function.

First the output voltage from the source must be capable of driving the headphones to full output. If this condition is met, then the output impedance of the source must be compared to the impedance of the headphones to be sure there is adequate drive.
Assuming the HD650’s we have a 300 ohm impedance and a sensitivity of 97dB/mW which roughly means you need about 20mW of power to drive them.

http://nwavguy.blogspot.com/2011/09/more-power.html

so 20mW into 300 ohms requires 2.5V so you first must be sure your source can deliver that. Next you need to make sure that your source has an output impedance low enough to drive that 300 ohm load. You are essentially looking for a worst case situation so a 3:1 ratio is a good place to start. If your source meets these two “minimums” then the autoformer is a good option since when you attenuate, the excess voltage gets converted into current which quickly drops the output impedance of the source. 6dB of attenuation would drop a source with a 100 ohm output impedance to 25 ohms and 12dB would make it 6 ohms.

dave

JohnG
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Re: Slagle Autoformer

Post by JohnG » Sun Apr 15, 2018 2:54 pm

AVC arrived during the week gone, connected it up today and A/B Compared it to a Stepped Attenuator.
A WAD K5881 was the power amp, the AVC performed exceptionally well and remained in the set up.
A difference that was very obvious to me as a comparison to other pre amps used, was that there was a lot more head room and expanded soundstage, with vocals and instruments being very clear with a space between them using the AVC.

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IslandPink
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Re: Slagle Autoformer

Post by IslandPink » Tue Apr 17, 2018 4:22 pm

Very please to hear it worked out so well.
Dave's stuff is world-class though, good to see he gave you such full advice, that's what I expected :D
White flags of winter chimneys waving truce against the moon ( Mitchell )

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