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It is currently 09 Dec 2018, 22:23

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PostPosted: 01 Sep 2018, 08:51 
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Wired not tested. Big yellow caps are Jensens copper paper and oil with silver leads. The big brown, 4uf Russian PIOs for the HT.
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PostPosted: 03 Sep 2018, 08:59 
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Grace's maiden voyage. A hell of a lot of gain. Can someone tell me the advantage of a totem pole preamp like this one. And please tell me the output impedance.
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PostPosted: 03 Sep 2018, 12:46 
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mwhouston wrote:
Can someone tell me the advantage of a totem pole preamp like this one.
The entire class of actively loaded (or "Shunt Regulated" as V.J. Cooper put it) amplifiers was developed to drive "difficult" loads with large voltage swings. Usually "difficult" meant relatively low impedance, highly capacitive loads driven at medium radio frequencies. In short, the topology was developed to address some very specific problems associated with high power television transmitters. Even though they are widely used today in certain audio amplifier designs, this is not for what these circuits were designed.
mwhouston wrote:
A hell of a lot of gain.
I'm not surprised by this at all. At your operating point, µ ≈ 20.5 and rp ≈ 8.0kΩ. This puts the amplification at roughly 13.7 v/v or 22.7dBv. Used as a preamplifier, a normal line level signal (+4.0dBu, 1.8dBv, or 1.23v-RMS) is going to be amplified to 26.7dBu, 24.5dBv, or 16.8v-RMS. This output level will need to be heavily attenuated to prevent overdriving most audio power amplifiers.
mwhouston wrote:
And please tell me the output impedance.
The output impedance is simply the parallel combination of the upper tube path and the series combination of the lower tube path and the bootstrapped upper cathode resistor. At your operating point, this puts the output impedance at about 5.46kΩ.

So more importantly, how does she sound?

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PostPosted: 03 Sep 2018, 17:18 
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Matt, thanks.

On a very brief listen all good. With my now 92db efficient 12" woofers and 150W Class D power amp if I get down on hands and knees, ear to the woofer, there is a low level hum.

Today I will get the chance to have a better listen. I was looking for a simple cct. which used both triodes and this one came up. I didn't use a Ck figuring I didn't need a huge amount of gain but got it anyhow.

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PostPosted: 03 Sep 2018, 23:05 
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Matt, I have the pot on the output as per the cct. but no Ck. Is the output impedance still 5.46K?

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PostPosted: 04 Sep 2018, 18:15 
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mwhouston wrote:
I have the pot on the output as per the cct. but no Ck. Is the output impedance still 5.46K?
Humph! :(

I didn't see the pot on the output. Natural Zo of the SRPP stage w/ Ck is 5.46kΩ. Natural Zo of the SRPP w/o Ck is more like 6.10kΩ. However, as we know from previous discussions on this thread, things are not quite so simple with the volume control on the output.

With the volume control the output, the output impedance varies as a function of the volume setting. For the version without Ck, it looks like this.
Attachment:
Screen Shot 2018-09-04 at 4.20.34 PM.png
The version with Ck looks almost identical; the peak impedance is just a little lower. Not enough about which to worry.

Hope this helps.


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PostPosted: 04 Sep 2018, 18:48 
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Playing up at volume yesterday the pot was no more than 60%. That makes the output impedance 15K. The ClassD modules have an input impedance of 20K. Is this a good match? I don't see it that way.

In the past with the pot on the output on 12AX7 4Ss I have heard fixed outputs from CD players over driving the input. On one preamp I moved the pot back to the front. On this preamp I've not heard any overdriving.

Thanks Matt once more.

How does it sound? Very new large output caps will take a while to settle. I feel, at this point, an element more clarity and maybe more body to the music. I need some better ears for an A/B.

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PostPosted: 04 Oct 2018, 11:17 
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is the ground lift switch an SPST type toggle?


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PostPosted: 21 Oct 2018, 01:03 
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If you want to follow "Grace" the Psvane 6SN7-SE preamp go here:http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6364

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PostPosted: 21 Oct 2018, 12:45 
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Ja Patt wrote:
is the ground lift switch an SPST type toggle?
Yes. "Connected" or "Not Connected" is the functional description of the ground lift switch. SPST is the minimum type required.

See description in the schematic on this post: http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3266&start=290

Also note that I actually used a SPDT switch and just left one side unconnected.

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