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 Post subject: 6SN7 line preamp.
PostPosted: 13 May 2022, 09:44 
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Joined: 03 Jan 2017, 20:59
Posts: 48
Hello. I currently use the 4S preamplifier on my set-up and want to build a line preamp using 6SN7 or 6SL7. I found this one interesting, other suggestions? It need to drive a Bryston 10B who have a input of 20k this one have a output of 650 ohm. Regards.


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 Post subject: Re: 6SN7 line preamp.
PostPosted: 13 May 2022, 11:00 
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Joined: 06 Apr 2009, 10:08
Posts: 1882
Location: US Pacific Northwest
It looks like it should be fine. I would make a few changes to keep it simple.

The power supply is designed such that current spiking on the secondary winding will be VERY high. I would reduce the first capacitor after the primary rectification bridge to about 22µf (but not more than 33µf) This will tame the current spikes and keep the transformer much cooler and much quieter.I would also reduce the capacitor after the 10H choke to about 100µf. Again this is to tame the overall current flow and prevent spiking.

I would also get rid of the filament winding rectification. The 6SN7 has excellent heater isolation and you will likely never hear any difference. Just used a nice 20AWG twisted pair to route the heater supply. The 10,000µf cap across the rectified filament winding will cause the same spiking issues I mention above.

I also think the OD3 is serious overkill for this circuit. The current draw from the preamp is very constant. A simple dropping resistor to get the voltage you want is more than sufficient.

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Matt
It's all about the Glass!
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 Post subject: Re: 6SN7 line preamp.
PostPosted: 20 May 2022, 21:16 
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Joined: 13 Feb 2013, 11:02
Posts: 65
Location: Canada
This will work, but there are a number of problems.
The first stage is running at less than a milliamp. The plate resistor is quite a bit too big, and the cathode follower is only running about 2mA. Also too low.
Do you really need all that gain too. Probably around 22 - 24dB.
Lose the OD3, run the preamp at around 250V, and drop the plate resistor to 47k. You can probably raise the cathode resistor on the follower to about 33k.
You might also think about adding some feedback just to drop the gain a bit.

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Dan Santoni
DTS Audio Electronics
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