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PostPosted: 23 Apr 2015, 17:08 
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Greetings, this thread started out as emails between myself and Matt. We are going to carry on the conversation in the forum so others can chime in and also for those interested in a 4S with a cathode follower output stage.
Cheers, Gio
~~~~~

Hi Matt,

I'm in need of a low gain tube preamp. I was thinking 2-stage 12AU7 with EZ81 PS - so essentially a 4S preamp but with a cathode-follower output stage. I was wondering if you could do a design for me?

Cheers,
Gio

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PostPosted: 23 Apr 2015, 17:14 
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Gio, for your preamp. I have a design I’ve drawn up for a 4S preamp with a nice 12AU7 cathode follower output. The overall output impedance is Ro=1.43kΩ, Co=0.65pf. This should be low enough to drive just about anything. I have included a formula for choosing the final coupling capacitor based on desired low end rolloff. It is a function of the input impedance of the equipment to which it’s hooked up. You should be able to choose a value that suits your needs. I know that you said “low gain” but the cathode follower stage is such that even a 12AX7 at full output cannot overdrive the buffer. As such, the preamp is totally free of blocking regardless of the preamp tube used in the gain stage. I also moved the volume control to the gain stage input to simplify the design. I don’t know whether you need a control in this unit, so whether you use it is up to you. You’ll note that I have the B+ listed at 300v. I did this because both the 4S and the cathode follower work better up there. But in reality, you could go anywhere from 250v to about 330v on the power supply without any issues.

Take a look at the attached schematic and let me know what you think.
Attachment:
4S-Buffer Preamp.jpg

Talk to you soon.
Matt


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PostPosted: 23 Apr 2015, 17:19 
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Hi Matt,

I'm finally getting time to start thinking about the low gain preamp project again. Thanks for sending the schematic and notes.

I have one comment/question regarding valve separation. I had thought about 4S+CF but the reason I went to thinking 12AU7+12AU7 was because I don't need the high gain, and I thought lower crosstalk is achieved by keeping the left and right channels in separate envelopes. I was thinking one bottle for each channel for best performance.

Yes, I will need a volume control. I guess I have another question. For best performance, what is the best location for the volume control? I don't mind it after the gain stage so I am not amplifying pot noise. The input source would be a CD Player or DAC and the preamp would drive a DIY FirstWatt F5 amplifier with Input Impedance 100k.

Thanks,
Gio

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PostPosted: 23 Apr 2015, 17:21 
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Gio;

As to the channel to channel architecture, there are two ways to go. The first is obviously to use one tube for L+R preamps and one tube for L+R buffer. The advantages in this case are the ability to use different preamp tubes. The disadvantage is the potential for some low level crosstalk between channels. However, I believe that if the B+ supply is split with separate final filters for left and right channels then you shouldn’t get any channel crosstalk; but that’s just my guess as I haven’t tried to measure it. The second choice is to do as you suggest and go with 12AU7s and use one tube (preamp + buffer) for each channel. The advantage is that with split supply this virtually guarantees near zero crosstalk. The disadvantage is that the only gain selection you can do is by bypassing or not the cathodes of the preamps (≈+/-6dBv). You could go either way. My guess is that you probably couldn’t tell the difference.

I have designed a tone control preamp I’m going to build and have done this same trade in my head about three times. I can’t decide on which way to go, but I am slowly going to the separate tubes for preamps and buffers approach. This is mostly because I can’t force myself to give up the versatility of the 4S preamp architecture for swapping tubes.

The choice for the volume control is a little more problematic. Again there are two options. The first is exactly as I have drawn the circuit with the volume pot on the input. The advantage of this design is that the potentiometer only handles low level ac signal. The downside is it’s noise figure dominates the preamp. The other option is to replace the load resistor of the preamp stage with a 100kΩ potentiometer, the wiper of which is connected to the buffer stage through the 0.1µf coupling capacitor. The advantage here is that the noise figure of the control is now integrated with that of the preamp (i.e. it should be better than the control in front of a preamp stage) and control noise (if any) is no longer in the grid circuit so it is not amplified by the preamp. But there are a few disadvantages. First, whereas the noise figure is better than the first option, the placement of what is likely a carbon load on the preamp raises the noise figure of the stage significantly from what it would be with a metal film resistor load. This means that the noise figure improvement from moving the gain control is not nearly equal to the gain of the preamp. The second is that the potentiometer will now be operated with a significant DC bias (≈300v). This means that the potentiometer must be made to handle this with proper isolation. The PEC KK series will do. They have high insulation resistance, 900vRMS dielectric strength at sea level, and an operating voltage range of 500v. One like this should work nicely (http://www.digikey.com/product-detail/e ... -ND/770961) but it is a little pricy at $31.26 USD.

The only other unknown is the output coupling capacitor. Using my 5.3Hz rule with the 100kΩ amp input impedance, the calculated value is 0.41µf; A 0.47µf value should work just fine. If you are driving a lower input impedance amp, the value will have to grow to keep the low end from suffering.

I realize that I’ve answered your questions with more options. But so much of these types of build decisions rotate around such trade offs that I really have done about the best I can to answer your questions. Read it over a few times and let me know if you have any further questions.

Regards.
Matt

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PostPosted: 23 Apr 2015, 17:30 
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Hello Matt,

Thank you for the detailed explanation.

That is how I understood crosstalk too. The only time I've been able to actually hear crosstalk is when there is no music in one channel and you can very faintly hear the other channel bleeding through when at very high volume, so I agree it is inaudible at real listening levels.

I had overlooked the DC bias on an inter-stage potentiometer and that for me is enough of a reason to keep it out of there. I use Alps Blue Velvet conductive plastic pots (~$15). I have no idea what voltage the pots can handle. I am under the impression that the conductive plastic pots are quieter than carbon, but I don't know that for fact.

I guess this still leaves the option of placing the potentiometer at the output of the preamp. Can we use a 100k pot at the output?

Cheers,
Gio

~~~~~
end of emails :cop:

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PostPosted: 23 Apr 2015, 18:57 
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Gio wrote:
I guess this still leaves the option of placing the potentiometer at the output of the preamp. Can we use a 100k pot at the output?
This is an option, but it kind of defeats the entire purpose of installing the buffer. With just the 250kΩ termination resistor, the output impedance of the circuit is ≈1.4kΩ. This is low enough to drive just about anything.

However, If you replaced the 250kΩ resistor with a 100kΩ volume pot, then the output impedance of the circuit would vary with volume setting from about 1.5kΩ at very low settings to about 25.35kΩ at around 85% volume and then falling again. It would look something like this:
Attachment:
output_impedance.png
It also makes the selection of the output coupling capacitor problematic as low end frequency response will now be a function of volume pot setting as well as the input impedance of the down stream device.

Overall I don't recommend putting the volume control on the output in this instance.

Personally I would have no reservations about using an Alps Blue Velvet at the input to this circuit.


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PostPosted: 26 Apr 2015, 13:43 
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Matt, this is a great extension of the 4S preaml because it allows builders to drive SS amp which generally have low inout impedance. The Class Ds I have been building have a 22K input impedance. With this preamp able to drive I into such low impedances it becomes a perfect partner for a Class D power amp.

I am yet to start the 1500W Class D but had thought about making it an integrated. A preaml such as this may just be the answer. Having the pot upfront will make a lot of DIYers comfortable. I could see the wisdom of the pot on the rear some years back but many don't like it. And what a great tube to use in a simple two stage tube preamp. There are some great NOS tubes out there. Thanks for the great cct.

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PostPosted: 27 Apr 2015, 19:44 
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Ok, I am good with having the potentiometer up front at the input. Thanks for explaining.

For the power supply, I have an old Hammond 269GX (225V-0-225V 65mA + 6.3V @ 2.5A) I would like to use. That should bring us quite close to a 300V B+. The modern 269GX has ratings 450V C.T. @ 75ma. 6.3V @ 2.5A.

For the choke I have a Hammond 156L (5H, 135 ohms, 75mA, 400VDC) that should be suitable.

I also like your suggestion to add an additional separated filtering stage.

Mark - I'm gonna give this one a go.

Cheers

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PostPosted: 27 Apr 2015, 19:57 
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I was thinking the same thing. But I would use my two tranni reversal trick to come up with the 300V.

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PostPosted: 28 Apr 2015, 13:40 
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Hi Folks, I've been following this along a bit largely to see how alternatives to some things I cooked up compare. The attached screen shot is of a basic version of the Forewatt. Performance wise it may be more what Gio needs. It is very quiet, easy to build and the Alps 100K works great at the input. A good build can get a very high S/N and the sound is quite transparent. With a Zo in the 2K range (with cathode bypass) to the mid 3K range (with out it) it will easily drive the 20K load. Lots of head room to boot.

Good listening
Bruce
Attachment:
Capture_04282015_113354.jpg


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