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Old 6th May 2015, 07:46 AM   #1
knutn is offline knutn  Norway
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Default Bride of Zen cascoded

I am thinking of using the Bride of Zen as a combined preamplifier and voltage amplifier to drive my power buffer (with no voltage gain). Therefore, I have to place my 50k volume potentiometer before the BoZ. I am afraid the high frequency impact is too large with this potentiometer without cascoding.

So, has someone tried a cascoded version of the Boz?
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Old 6th May 2015, 11:31 AM   #2
Zen Mod is offline Zen Mod  Serbia
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consiidering that you have 30V at output node , make that zener 18V , re-arange bias circuit for lower one

in any case, lower mosfet will work better with more Uds
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Old 6th May 2015, 11:55 AM   #3
knutn is offline knutn  Norway
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I see, is there any "sweet spot" for the lower MOSFET?
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Old 6th May 2015, 01:35 PM   #4
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I would start from there ..... and up the current later

to do that , decrease both R load (1K) and Rs , to roughly maintain gain and voltage in output node

then fine tune voltage in output node with bias voltage
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Old 6th May 2015, 02:00 PM   #5
juma is online now juma  Germany
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Quote:
Originally Posted by knutn View Post
...So, has someone tried a cascoded version of the Boz?
I did, and it didn't bring me much happiness either...
I suggest attached circuit:
Input buffer (Q1,Q2) gets you rid of the input cap and isolates the pot so you don't have to worry about it.
Amplifying MOSFET (Q3) works with local Schade feedback in triode mode, loaded with CCS (Q4,Q5).
Zout is about 150 R .
Gain (about 20dB) is set with R7/R5 ratio and half of the PSU voltage at Q3's drain is set with R6 (depending on Q3's Vgs/Id characteristic).
This circuit has much better PSRR than original. It will deliver more than 20V_peak of clean swing at the output and is linear up to almost 100kHz.
Of course, it can be further improved in many ways - first thing would be to use 2sk2013 instad of IRF610 for Q3.
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Old 6th May 2015, 06:07 PM   #6
knutn is offline knutn  Norway
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The reason I wanted to use the BoZ was to make a 2 stage power amplifier without any feedback loops and compare it to my earlier much more complicated amplifiers.
The solution with a source follower input instead of the cascode can be an option, of course, though.
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Old 6th May 2015, 08:43 PM   #7
flg is offline flg  United States
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This might be an option?
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Old 8th May 2015, 08:08 AM   #8
knutn is offline knutn  Norway
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I have simulated Juma's circuit (without the JFETs), and I must say I am impressed. The topology is very similar to the Zen amplifier. So I think I will give it a try.
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Old 8th May 2015, 09:22 AM   #9
juma is online now juma  Germany
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Yes, Schade's "trioding" is great, but it comes at a price: the input resistance is determined by the value of R5 and it needs to be low - otherwise the frequency responce will be compromised in upper range and that's why the buffer (Q1, Q2) is needed.
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