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 Post subject: Re: 6SN7 preamp
PostPosted: Tue Jul 18, 2017 2:27 am 
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Muriel
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Hi,

felipe wrote:
I'm not using feedback and not 100K pot (I'm using Intact Audio AVC).


Well, this is probably not an approach I'd take. Your input impedance will be highly reactive (essentially the AVC).

Consider making a stage will feedback, just enough gain to get the volume levels you want.

This should have a pretty low output impedance (parallel tubes if not low enough) and give the AVC a consistent low source impedance.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Tue Jul 18, 2017 7:59 am 
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Cow

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Location: Catalonia New Europe State
Hi,

Thanks for advice. I'm using OPT EDCOR - GXSE5-15K no coupling cap at the output. What's the input impedance?

TIA
Felipe


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 Post subject: Re: 6SN7 preamp
PostPosted: Wed Jul 19, 2017 11:10 am 
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Muriel
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Hi,

felipe wrote:
I'm using OPT EDCOR - GXSE5-15K no coupling cap at the output. What's the input impedance?


I don't think your current design has much relation to what I published. So I can't really help you. Maybe if you draw out a full schematic of what you have and get the datasheets for all the various transformers, we can have a look how this circuit may perform...

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Wed Jul 19, 2017 2:05 pm 
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Cow

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You are right, I will post schematic used & transformer datasheet.


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 Post subject: Re: 6SN7 preamp
PostPosted: Thu Jul 20, 2017 8:19 am 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
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Location: Catalonia New Europe State
Information requested (I use as headphone amp for Sennheiser HD600 = 300‎Ω), SSHV2 is a high voltage shunt regulator made with sand:

[url][url=http://subefotos.com/ver/?b06431fc592d726586eb1c973d23e863o.jpg]Image[/url][/url]

AVC Intact Audio stacked for 20H. It allows attenuation from 0dB to -42db in -3dB steps. It has an 80% nickel core.
Image

GXSE5-15K-600‎Ω
Wattage 5W
Max. Current 100mA
Primary (input) Impedance 15K Ohms
Screen/Grid Tap 40%
Secondary (output) Impedance Depends on model.
Frequency Response 40~18K Hz., <1dBu
THD+Noise <0.1% @ 1K Hz.
Bobbin Material Nylon 6/6 GF-30
Flamability Rating Class B 130°C
Core Material M-6 29 ga. grain oriented lamination steel
Secondary Insulation (transformer) Clear air dry polyester varnish
Termination 0.187" (3/16") quick disconnects
Mounting Zinc plated steel channel/frame
Weight 1.2 lbs.
Compliance RoHS & REACH

If it's a way to improve please let me know?

TIA
Felipe


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 Post subject: Re: 6SN7 preamp
PostPosted: Fri Jul 21, 2017 8:31 am 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
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Location: Catalonia New Europe State
T if you know how to improve please let me know?

TIA
Felipe


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 Post subject: Re: 6SN7 preamp
PostPosted: Tue Jul 25, 2017 2:27 am 
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Muriel
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Hi,

felipe wrote:
Information requested (I use as headphone amp for Sennheiser HD600 = 300‎Ω), SSHV2 is a high voltage shunt regulator made with sand:

Image

AVC Intact Audio stacked for 20H. It allows attenuation from 0dB to -42db in -3dB steps. It has an 80% nickel core.


Ok, lets start at the input. You use an AVC, which is a tapped inductor. It's impedance is extremely dependent on frequency.

The AVC with 20H will have an input impedance of 2.5kOhm at 20Hz, but 25kOhm at 250Hz. Due to parasitic winding capacitance the impedance will peak at a quite high value due to resonance and then will fall again towards higher frequencies. I belaboured these relationships in more detail in the App-Notes for the S&B TX-102 TVC, which are gone from S&B's website but remain on the internet archive:

https://web.archive.org/web/20070317065510/http://www.stevens-billington.co.uk/page102.htm

The upshot is that the drive impedance to your AVC should be no higher than 1/10 of the 20Hz impedance, or 250 Ohm.

If capacitor coupling is used the coupling capacitor to the AVC should be no less than 50uF.

If you restack the AVC for the 190H maximum all impedance values at 20Hz go up by 10 and the minimum coupling capacitor value is cut into 1/10th.

Continuing at the backend...

Your Output Impedance will be around 80 Ohm plus the copper losses, which Edcor fails to state.

Further the Edcor Transformer you use has limited bandwidth and pure steel laminations, which is not really that conductive to subjective sound quality. I would probably seek to use a Radiometal 36 core with 36% or so of nickel & a suitable air-gap and an improved winding structure to get 10Hz - 40kHz @ -1dB which is not really challenging.

felipe wrote:
If it's a way to improve please let me know?


I would start by moving the AVC to behind the output transformer. This way it gets a known, very low impedance and can attenuate Tube noise etc as well.

If you insist in keeping the AVC in front, I'd restack for maximum inductance. Be very careful not to bend or stress the Permalloy Lams as this can mess up their magnetic properties. And only use non-magnetic screwdrivers, screws etc.

If you move the AVC to the back you may want to bump up the bias to 6V (2 batteries in series) and increase the HT to 150V get back the Anode current.

With 2V in and the AVC set to max you will get around 5V out into your Headphone and appx 5% H2 (eyeballing the plate curves in the datasheet).

Of course, the harmonic distortion depends solely on the input voltage, more means more HD, less means less, classic triode 2/3rd square law.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Sun Jul 30, 2017 11:59 am 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
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Location: Catalonia New Europe State
Phono preamp have opt cap 3,9uF & DAC have opt cap 10uF, will be enough the phono opt cap?


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 Post subject: Re: 6SN7 preamp
PostPosted: Sun Jul 30, 2017 12:02 pm 
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Cow

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The problem to use the AVC behind is that it's mounted in a separate box like passive preamp because I use the AVC for the #26 preamp & your preamp.


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 Post subject: Re: 6SN7 preamp
PostPosted: Sun Jul 30, 2017 4:26 pm 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
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Location: Catalonia New Europe State
If I put the AVC behind what's the optimum AVC inductance?


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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Jul 31, 2017 2:26 am 
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Muriel
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Hi,

felipe wrote:
Phono preamp have opt cap 3,9uF & DAC have opt cap 10uF, will be enough the phono opt cap?


You better measure the LF frequency response. And you better talk to intact audio, sureley they took your money and should suport their product.

felipe wrote:
If I put the AVC behind what's the optimum AVC inductance?


As drive impedance is low, but levels are high stacking for highest level and lowest inductance (e.g. what you currently have) would be my choice. But again, better talk to intact audio.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Jul 31, 2017 7:34 am 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
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The gain lowered a lot stacked for 150H.


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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Jul 31, 2017 7:35 am 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
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Location: Catalonia New Europe State
Kuei Yang Wang wrote:
Hi,

felipe wrote:
Phono preamp have opt cap 3,9uF & DAC have opt cap 10uF, will be enough the phono opt cap?


You better measure the LF frequency response. And you better talk to intact audio, sureley they took your money and should suport their product.

felipe wrote:
If I put the AVC behind what's the optimum AVC inductance?


As drive impedance is low, but levels are high stacking for highest level and lowest inductance (e.g. what you currently have) would be my choice. But again, better talk to intact audio.

Ciao T


You're right, better ask to Dave.


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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Jul 31, 2017 2:35 pm 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
Posts: 193
Location: Catalonia New Europe State
Kuei Yang Wang wrote:
Hi,

You better measure the LF frequency response.

Ciao T


How to measure?


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 Post subject: Re: 6SN7 preamp
PostPosted: Thu Aug 03, 2017 4:37 am 
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Muriel
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Joined: Thu Jul 26, 2007 5:45 pm
Posts: 1179
Hi,

felipe wrote:
Kuei Yang Wang wrote:
Hi,

You better measure the LF frequency response.

Ciao T


How to measure?


About any way you care to. I might use an Audio Precision System 2 as I have one in the lab, but in a pinch any tone generator and 'scope will do.

Ciao T

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