Hello there!
Im thinking of building Merlin's phono preamp, and I have thought of adding and extra gain stage at the end so it can drive my Kegger KT88 amplifier.
15db of gain would be more than enough.
I have a bunch of 6922 to use. So I'll choose that tube for the extra stage.
First question, what's best, potentiometer at begginning of extra gain stage or after (at the very end of the preamp)?
I know the position of the volume control would alter the output impedance on the later, but.. do not think would be a problem with the 1M input impedance of the Kegger amplifier.
I have also though of adding a BALANCE control (the lineal balance/log volume control. suggested by both Merlin / Jones).
I believe those have a attenuation of near 9db in the center position, right?
Thanks for your help!🙂
Im thinking of building Merlin's phono preamp, and I have thought of adding and extra gain stage at the end so it can drive my Kegger KT88 amplifier.
15db of gain would be more than enough.
I have a bunch of 6922 to use. So I'll choose that tube for the extra stage.
First question, what's best, potentiometer at begginning of extra gain stage or after (at the very end of the preamp)?
I know the position of the volume control would alter the output impedance on the later, but.. do not think would be a problem with the 1M input impedance of the Kegger amplifier.
I have also though of adding a BALANCE control (the lineal balance/log volume control. suggested by both Merlin / Jones).
I believe those have a attenuation of near 9db in the center position, right?
Thanks for your help!🙂
Usually the volume control works out best at the input of the line stage, especially if the gain of the
line stage is fairly high (like yours). Overloading the line stage input in this case is not possible.
An advantage of having the volume control after the line stage is that, at lower volume settings,
the inherent noise of the line stage will be reduced along with the signal level. With well designed
circuits this should not be necessary.
Some balance controls, such as those with switched resistors, need not have a loss at the center setting.
line stage is fairly high (like yours). Overloading the line stage input in this case is not possible.
An advantage of having the volume control after the line stage is that, at lower volume settings,
the inherent noise of the line stage will be reduced along with the signal level. With well designed
circuits this should not be necessary.
Some balance controls, such as those with switched resistors, need not have a loss at the center setting.
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You have a good implemetation in dynaco PAS4
http://n.manet.nu/pas4-klon/Dynaco-pas4.pdf
I have a analyser output from this stage :
http://n.manet.nu/pas4-klon/pas4-spektrum-line.jpg
( An unedited picturer of my 3580A analyzer. This is 380mV into line and 1V out )
it uses a single E88CC/6922 per channel, see the attached schematic.
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Thank you for answering. I haven't thought about overloading, yes, I think the volume at the beginning is best then.
Thank you answering!
I have checked the schematic you supplied. It seems it uses two extra stages after the phono stages, not just one as I would prefer.
EDIT: whats the purpose of the second stage? First thought it was a cathode follower, but it is not.. is it SRPP?
Other thing got my attention is the 50K pots both for balance and volume. Wouldnt the resistance drop too much with those?
Thanks!
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According to merlin blencowe this is a ü-follower, a variant of SRPP.
It has low output impedance and good dist properties.
The pots can be chosen to what ever you like, they are at the input and won't
affect the workings of the stage ( except for the nfb ). The dynaco riaa stage
has low impedance output, so has the merlin riaa, they both will work fine with 50k pots.
pots are a mess to get hold of, 50k is one of the easiest , 100 k might nearly as easy to obtain.
The amp has very good properties and i do recommend it !
peter h
Thank you again, Petertub.
Actually I dont want to copy a whole design.
About that mu follower, perhaps it was used that way to reduce output impedance so it can drive low impedance loads (SS amps).
Remember I would use the preamp with the Kegger (a 1M input impedance) amp. Correct me if I am wrong but, do I really need a buffer?
Can I just use a simple triode gain stage. Hasn't done the math yet, but I estimate the output impedance not to be that high to drive it. Sure below 60k. Remember I would use a 6922.
Thanks for you advice!
Actually I dont want to copy a whole design.
About that mu follower, perhaps it was used that way to reduce output impedance so it can drive low impedance loads (SS amps).
Remember I would use the preamp with the Kegger (a 1M input impedance) amp. Correct me if I am wrong but, do I really need a buffer?
Can I just use a simple triode gain stage. Hasn't done the math yet, but I estimate the output impedance not to be that high to drive it. Sure below 60k. Remember I would use a 6922.
Thanks for you advice!
With 1M input (load) impedance the problem won't be Zout from your gain stage, it will be how to keep the gain under control, and noise along with it.
6922 / 6DJ8 / ECC88 / E88CC as a common cathode amplifier will have gain of about 20X to 25X. (25dB or so)
The phono stage will provide about 40dB of gain.
You need 65dB of gain from the cartridge to the power amp?
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6922 / 6DJ8 / ECC88 / E88CC as a common cathode amplifier will have gain of about 20X to 25X. (25dB or so)
The phono stage will provide about 40dB of gain.
You need 65dB of gain from the cartridge to the power amp?
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For phono cartridge to power amp I need total 55dB.
I thought about using other source as well and not only phono.
Remember if I add a balance control I have a loss of 6 to 9 dB in the center position..
So you say I would have noise issue with a simple triode gain stage.. I see.
I thought about using other source as well and not only phono.
Remember if I add a balance control I have a loss of 6 to 9 dB in the center position..
So you say I would have noise issue with a simple triode gain stage.. I see.
I don't mean putting a simple triode stage in the chain will automatically give you a noise problem. I think putting together a chain that has too much gain might give you a noise problem.
Which MerlinB phono preamp do you have? The one with PC97 and 12AU7? Or one of the two based on 12AX7?
Which MerlinB phono preamp do you have? The one with PC97 and 12AU7? Or one of the two based on 12AX7?
Havent built any yet.
In his book he shows the PC97 version.
But in his site he updated it to support a couple of different tubes. -> will go with that one.
In his book he shows the PC97 version.
But in his site he updated it to support a couple of different tubes. -> will go with that one.
btw, my idea comes from this pre:
4S Universal Preamplifier for 12A*7 Tubes
It uses other tubes though..
4S Universal Preamplifier for 12A*7 Tubes
It uses other tubes though..
Are you aware of merlins circuit boards ?
The Valve Wizard
A small sum for the board, good instructions comes with it.
Merlin offers two PCBs for phono preamps on the Valve Wizard site. Neither of them is for the PC97 circuit he shows in his high-fidelity tube preamps book. The PCBs on the Valve Wizard site are for twin-triodes that use 9pin mini (B9A) sockets, like 12AX7, 12AT7, 6DJ8, etc.
I just built my own variation on the circuit on a pair of his Compact Phono PCBs (12AX7 with MOSFET source follower output). With a 12AX7 you can get 44dB to 48dB gain right there. For the source follower I'd use a MOSFET with very low Crss, like ZVN0545A, if you can find them. They have extremely low Crss.
The boards are quite nice, by the way. Very intelligently laid out.
I just built my own variation on the circuit on a pair of his Compact Phono PCBs (12AX7 with MOSFET source follower output). With a 12AX7 you can get 44dB to 48dB gain right there. For the source follower I'd use a MOSFET with very low Crss, like ZVN0545A, if you can find them. They have extremely low Crss.
The boards are quite nice, by the way. Very intelligently laid out.
I am still unsure to order him one or just build one myself.. Will see that later though, when I have the schematic finished. First I want to think about the extra triode gain stage.. how it can be noisy and if so, how I can overcome it without adding an extra triode. 😕
The first thing to do is decide how much gain you need from this extra triode stage.
There is a circuit known as an anode follower, which is a common cathode gain stage with a local negative feedback loop from the plate to the grid of the gain stage. Because it uses negative feedback to set the gain, you can adjust the circuit to any amount of gain you need, as long as the closed-loop gain is less than the open-loop gain of the common cathode stage itself.
Here's John Broskie's explanation of that circuit:
Anode-Plate Follower
You need to decide
1) How much gain you need the entire system to have
2) How much gain the phono stage provides
3) How much gain the add-on gain stage needs to provide
Total gain minus gain of phono stage equals gain of add-on stage
Atot - Aphono = Aaddon
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There is a circuit known as an anode follower, which is a common cathode gain stage with a local negative feedback loop from the plate to the grid of the gain stage. Because it uses negative feedback to set the gain, you can adjust the circuit to any amount of gain you need, as long as the closed-loop gain is less than the open-loop gain of the common cathode stage itself.
Here's John Broskie's explanation of that circuit:
Anode-Plate Follower
You need to decide
1) How much gain you need the entire system to have
2) How much gain the phono stage provides
3) How much gain the add-on gain stage needs to provide
Total gain minus gain of phono stage equals gain of add-on stage
Atot - Aphono = Aaddon
--
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Did you build it using Merlin's recommended parts values for ECC83, as shown in his parts selection flowchart?
http://www.valvewizard.co.uk/Phono_PCB_BOM.pdf
http://www.valvewizard.co.uk/Phono_PCB_BOM.pdf
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Hello.
I would need:
phono to preout of 55 to 60dB (I would assume Merlin phono has 40dB).
extra gain stage: 15dB to 20dB.
Remember that if I add balance control I would need an extra 6 to 9dB..
As I said earlier if I can accomplish that with just a common triode gain stage.. without a cathode follower or other type of buffer.. that would be great.