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 Posted: 12 Feb 2017, 13:40

Joined: 09 Feb 2017, 20:50
Posts: 3
Location: USA
I am new to DIY audio and I am interested in building the 4S as my first project. My goal is to control a SS amplifier with with a 47K input impedance and a 1.1 V input sensitivity. I am still trying to learn the basics but I understand that the 1.4K output impedance of this cathode-follower output stage circuit will work great with this amp. I am curious about how the preamp output voltage will react with the SS amp. Is the output voltage the same as the 4S without the the second stage? If so, what should I expect to happen with volume control? If the pre is outputting 7 volts max then it would stand to reason that with a 1.1V maximum amp input, the preamp will give me maximum volume with a small turn of the pot. How would, or should, the circuit be modified to bring the output to input voltages more inline?

I hope that my questions make sense. I am just starting to learn about audio circuits. I might need to dust off my basic circuit analysis text book from college to help bring me up to speed. Any input is greatly appreciated.

Thanks,
Jesse

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 Posted: 12 Feb 2017, 20:55
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1648
Location: US Pacific Northwest
jesseday2 wrote:
I am curious about how the preamp output voltage will react with the SS amp. Is the output voltage the same as the 4S without the the second stage?
The direct answer to your questions in that the voltage is about 10% (≈1dBv) lower with the cathode follower stage. This is small enough that you would never notice the difference.

Now lets talk about gains and losses. If you look at the schematic on the first page you'll notice three distinct sections to this preamp. They are 1) the volume control, 2) the 4S preamp itself, and 3) the cathode follower. Let's take them in turn.

A standard audio taper volume control is a variable attenuator. At 50% "volume", or at 12 o'clock the volume control has a attenuation of about 20dBv. Remember this number. So a normal volume control goes from about -40dBv when fully counterclockwise (but just barely conducting), to -20dBv at 12 o'clock, to 0dBv (i.e. no attenuation) fully clockwise.

The 4S preamp has a gain which is dependent on the tube used and dependent on whether the cathode is bypassed or not. A chart of these gains can be found on the 4S project page here. You'll see that a 12AU7 without cathode bypass gives about 20dBv of gain.

Finally, the cathode follower actually has a small loss (≈ -1dBv). Its entire job is to lower the output impedance so that the preamp can drive lower impedance circuits (e.g. solid state amplifiers).

So lets put it all together. If we use the 12AU7 (unbypassed cathode) referenced above, then it plus the cathode follower has an overall gain of about 19dBv. This means that the volume control range will go from -21dBv at the very lowest setting, to about -1dBv at 12 o'clock, to 19dBv fully clockwise. In short, prior to 12 o'clock, this setup is essentially an attenuator, and beyond 12 o'clock, it can give up to 19dBv (or about 9 v/v) of gain.

Now how your solid state amplifier responds is dependent on the level of the signal driving the preamp, the amplifier's input sensitivity, whether it has a volume control and how it is set, and the clipping level of the amplifier. If you have a device feeding the amplifier and everything is working fine. Then you insert this preamp (in the configuration discussed above) between the two, with the volume control at 12 o'clock, it should work the same. If you decrease volume it will, of course, get quieter, if you increase volume then at some point you may overdrive your amplifier. But exactly when that will occur is difficult to say without a lot more information on your setup.

Does this help?

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Matt

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 Posted: 13 Feb 2017, 21:29

Joined: 09 Feb 2017, 20:50
Posts: 3
Location: USA
Yes that helps. A great explanation on what I was hoping to understand. Thanks for the detailed reply. I have now read through the 4S forum and have learned a lot from you and the other posters. I now feel comfortable to start buying some of my components for the build. It may take me a little while to get everything I need. I will post some pictures of my progress and I am sure follow with more questions. Thank you again.

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 Posted: 13 Feb 2017, 21:56
 Editor

Joined: 28 May 2008, 21:53
Posts: 4586
I finally got a chance to get going with my build. This is my first build since I added a drill press to my tool collection so I'm looking to step up my enclosures as in the past they have been quite industrial and utilitarian.
I've got most of the drilling completed as you can see in the photo.

Attachment:
Drilled-Chassis.jpg

The cutout for the IEC receptacle was made with a Klien nibbler tool which worked rather well. For some reason I was expecting more difficulty with the steel chassis. The tube sockets were drilled with a bi-metal hole saw. I decided to put most of the connections and controls on the top of the chassis. Looking back, one regret I have is lining up the tubes - I should have pulled the rectifier tube closer to the transformer to shorten the length of AC wiring. The potentiometer will be located near the inputs/outputs and a shaft will extend between the tubes to the front plate.

I still have some small holes to drill for the LED and the tube socket bolts.

Matt - one question - in the schematic, is that one tube per channel or one tube per stage?

Cheers

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 Posted: 13 Feb 2017, 22:12
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1648
Location: US Pacific Northwest
Gio wrote:
Matt - one question - in the schematic, is that one tube per channel or one tube per stage?
Stick wth one tube per stage. This preserves the ability to use different preamp tubes if you want different gains.

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Matt

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 Posted: 14 Feb 2017, 03:53
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5226
Location: Australia
Looking good.

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 Posted: 14 Feb 2017, 04:03
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5226
Location: Australia
P.S. I don't like RCAs or knobs or switches on the top plate. There was stacks of room on the back. Doesn't mean it will not sound good it's more about appearance. It's just me.

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 Posted: 14 Feb 2017, 20:38
 Editor

Joined: 28 May 2008, 21:53
Posts: 4586
Thanks for the feedback Mark. Normally I would put the RCA and ground lift on the back and the power switch at the front. But since I was building a Universal preamp, I was thinking top mounted for easier access? For the power switch, I like that with the top mounting you eliminate a bunch of AC mains wiring. This power switch, Tx, fuse and IEC layout I think I will repeat in future tube builds. Definitely a new layout style for me, we will have to see how I like it in the end.

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 Posted: 14 Feb 2017, 20:40
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1648
Location: US Pacific Northwest
Now that this project is in process again I wanted to take a minute and discuss one interesting aspect of the design of which builders should be aware. On the schematic is a formula for choosing an output coupling capacitor based on the input impedance of the amp being driven and the desired low frequency rolloff point. The choice of capacitor value is not so straight forward as one may assume.

The main reason for building this project is to have a tube preamp which can directly drive amplifiers with lower input impedance; specifically to drive solid state amps. One of the differences between SS amps and tube apps is that (for the most part) when SS amps are overdriven, the do not transition to low impedance inputs like vacuum tubes do. Remember that vacuum tubes, when the grid is driven positive, draw grid current. This is, in essence, a sudden transition to a low input impedance. What this means is that when driving a SS amp there is no danger of the coupling capacitor driving a bias excursion and hence, block distortion. Since there is no problem with bias excursions and blocking the builder is free to choose as large a capacitor as desired (within reason). In this case, it is tempting to simply choose some large value of coupling capacitor and be done with it.

To chose a value, we start with the fact that the cathode follower has an output impedance of 1.43kΩ. Using the 4x rule, this means that you could drive an amplifier with an input impedance as low as about 5.7kΩ. Designing for this instance, one could choose a 2.2µf capacitor for -3dB frequency of about 10.3Hz. And as the input impedance of the amps increase, the low end response only gets better. Using this philosophy here is what the low end rolloff look like for amp with input impedances from 5kΩ to 45kΩ in 10kΩ steps.
Attachment:
2p2mmf cap.png

But here is the rub. Let's say that in the future you decide to attach this preamp to a nice new tube amp you built. One with an input impedance of 100kΩ. With the 2.2µf coupling capacitor, the low end response is going to be really great; almost flat. However, the bias excursion recovery time constant is now 160mS. This means that every time the preamp drives a bias excursion large enough to cause a blocking event, it will take almost a full second for the input stage to recover. And since it's a preamp which is amplifying the signal, the chances of this happening are rather high.

So what happens if we design for this case. If we want to keep the time constant less than 30mS (which is fast enough to not miss a beat in a 400BPM score) the capacitor needs to be smaller than about 0.41µf. Choosing the next lower standard value, 0.39µf, gives a time constant of 28.4mS, so everything is ok. Here are the low end rolloff curves for input impedances from 80kΩ to 120kΩ using the 0.39µf coupling capacitor.
Attachment:
0p39mmf cap.png

This looks pretty good and there should be no major problem with blocking distortion. But what about the performance with the SS amps? Here are the rolloff curves for the same low input impedance amps we saw above but with the new capacitor value.
Attachment:
0p39mmf cap 2.png

Now things don't look so great for the low frequency response of the SS amps.

Once again we find ourselves in the land of trade offs. Now, in reality, most solid state amps have an input impedance of at least 10kΩ. It is rather unusual to find a lower input impedance than this and actually, 20kΩ to 50kΩ is more common. We could chose a value that gives acceptable low end performance, say 0.56µf, and take what we get on the 100kΩ tube amp. In this case the low impedance rolloff curves look like this:
Attachment:
0p56mmf cap.png
... and the bias excursion recovery time for the 100kΩ tube amp is only 40mS which, while not meeting our target, is not bad overall. But there is one other option for those builders that want to preserve the maximum flexibility. You could do something like this:
Attachment:
Selector.jpg

This will allow the selection of which type of amp you want to drive, either low impedance solid state or high impedance vacuum tube, all with no loss of fidelity. The curves for this configuration look like this:
Attachment:
Switched_Curves.jpg
And the bias excursion recovery time is short enough all the way up to 250kΩ using the 0.22µf setting.

So hopefully people now have a better idea of how to go about choosing this capacitor value and maybe how to extend the design to make this preamp useful for both SS and VT amplifiers. As always, questions and comments are more than welcome.

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Matt

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 Posted: 16 Feb 2017, 17:30
 Editor

Joined: 28 May 2008, 21:53
Posts: 4586
Thanks for explaining that Matt. The concept of time constant is entirely a new consideration for me!

I had picked out a 0,47uF, but it sounds like for my application a 0.22uF will work better. I don't think I will ever need to drive low impedance, but I may just add the 1uF and switche(s) as well.

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