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Active load (totem pole) 6SN7-SE preamp
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Author:  mwhouston [ 13 Nov 2018, 17:20 ]
Post subject:  Re: Active load (totem pole) 6SN7-SE preamp

I thought I had ordered more stepper attenuators. It appears I haven't. The schematic says 100K pot on the output of the active load cct. But I only have 250K stepper. I can't see this being an issue but I realise I will have to turn the stepper up more to get the same volume.

It may be even better in as much as from no sound to the first step it is a tad loud for realy quiet background music. Of course there is nothing in between. The large 250K pot would make the first step a quieter one. If not successful I guess I can put a couple of 250K ohm resistors across the pot to bring it back to 125K.

Anyone see any problems with all of this? The preamp works into a 20K load so the bigger pot may help here also taking some load off the preamp.

Author:  ILoveHiFi [ 13 Nov 2018, 21:28 ]
Post subject:  Re: Active load (totem pole) 6SN7-SE preamp

I'd go with a 20k or 10k pot if its driving a 20k load, larger pots means theres larger series restance with the input of load that means limiting the rate at which internal capacitances can chrage up and possibly causing voltage drop at 20khz at exteemes saying using 1m pot into 20k load.

Putting resistances in parallel with the pot is not going to help your prolbem

Author:  mwhouston [ 14 Nov 2018, 00:37 ]
Post subject:  Re: Active load (totem pole) 6SN7-SE preamp

The preamp is up and going as of 5mins ago. It's over bright at the mo due to unburnt-in parts. I remember the first was the same. Those giant Jensen copper paper and oil caps take a few days to burn in.

I'm happy with the 250K pot .really can't tell the difference.

Author:  mwhouston [ 14 Nov 2018, 06:10 ]
Post subject:  Re: Active load (totem pole) 6SN7-SE preamp

I guess when two preamps are exacly the same in every way except one and they sound different it must be that one part making all the difference. The new pramp "Elite" sounds better than the first - Grace. It has the Grade A or low noise version of these Psvane globes. Can't wait for 50hours to pass so tubes and parts are burnt in. But I can tell the sound is better and not just a little bit.

There was one other change. I added a bi-pass 0.01uf poly cap across the big Jensen 0.47uf output cap. Normally I would do this as a matter of course but in Grace, the first build, I didn't. I always feel I get better HF response if I do this and the smaller cap provides an easier path for the HF signals. Not sure the science is right but each time I've done it I feel the sound improves.

Grace has been spoken for and that is why the next one has come about so quick. I'm thinking a third but with some RCAGE 5961s NOS tubes I was given. These tubes are nothing to look at but are sort after and the better ones not cheap on the net. Better order another Hammond chassis.

Author:  mwhouston [ 16 Nov 2018, 19:48 ]
Post subject:  Re: Active load (totem pole) 6SN7-SE preamp

Trust your gut. I have made this statement a few times usually in relation to finding something wrong with playback. In the new Elite 6SN7-SE preamp I added what I thought would be a 0.01uf snubbing cap across the larger output cap. A good practice I have used in other builds. But I accidentally put the cap from the output cap to the input of the preamp.

I felt while listening to music with this preamp over the last few days, that on rare occasions, there was some muffled distortion. I blamed the music. But trusting my gut that there was something wrong with the latest build I went investigating. Finding the small cap connected incorrectly I fixed it putting it across the 0.47uf output cap. But then I had no control over volume!?!?

With Grace working so well without this extra cap I cut it away. Now all good and the true clarity of these low noise tubes and the preamp really shines through. I often put up my mistakes to make others aware that regardless how long you have been in the game it is so easy to make mistakes. I've been into electronics for over 45years. It's too easy to make small mistakes especially for me as I don't often use a PCB which 'tend to show layout, parts placement and value.

So if it doesn't sound right and it may take a day or two to realise, then check it out. More often than not you'll find some simple mistake, e.g. 390K and not 39K. Done that one a few times.

Author:  mwhouston [ 18 Nov 2018, 15:22 ]
Post subject:  Re: Active load (totem pole) 6SN7-SE preamp

I found the cause of the uncontrollable volume. It appears the step attenuator was failing. I would image some sort of contact breakdown or misalignment.

Fortunately I had a spare and now all is good. I may try the snubber cap back across the output cap. In theory it is good practice and somethings I always do. Also I discovered the 24V 1A wall wart voltage drooping under load. That has now been replaced with a 24V 3A tranni. A little more HT but had to up the limiting resistor which feeds the filaments.

Author:  ILoveHiFi [ 18 Nov 2018, 16:20 ]
Post subject:  Re: Active load (totem pole) 6SN7-SE preamp

If you want to do a comparison seeing if your 250k output pot is draging you back in sound quality then I would just turn the pot on max volume and rely on computer digital volume control to lower volume and test the sound quality.

Took me quite some time to realise but I really like the idea of buck or even buck boost for heaters voltage, even the very small ones heatsinkless can handle 1.2A per piece at 6.3v very nice. Always fixed voltage and good.

Buck is when you have like 3v or more than 6.3v dc to play with to reduce voltage and buck boost is for when you want to get 6.3v dc when your supply voltage is from 2 to 9.3v dc.
Boost aren't usually helpfull because most transofmers after rectifying will have more than 6.3v dc or equal you can't reduce voltage in boost converter.
Buck boost can keep voltage constant regardless of input voltage.

Author:  mwhouston [ 27 Nov 2018, 20:00 ]
Post subject:  Re: Active load (totem pole) 6SN7-SE preamp

Now with new premium EIZZ step attenuator. It uses AMRS resistors and apart from the colour the rest is the same. @$135AU a piece you would have to weigh the merits. Each to his own.

Image to follow.

Author:  Gio [ 28 Nov 2018, 22:36 ]
Post subject:  Re: Active load (totem pole) 6SN7-SE preamp

Hi Mark,

What socket and plug are you using for the power connectors from the power supply to the preamp?

Author:  mwhouston [ 28 Nov 2018, 23:07 ]
Post subject:  Re: Active load (totem pole) 6SN7-SE preamp

Gio, this may shock a few but XLR male and female. Supposedly XLR is only designed for low voltage but if you have ever had one apart you will see how much insulation is between pins etc. There is female on the PS end and a male socket on the preamp.

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