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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really)
BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really)
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Old 11th February 2019, 07:05 PM   #761
maartn is offline maartn  Netherlands
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Join Date: Jun 2017
Hi Hans, not sure if I understand that. In my current set-up J6 is connected to +/-16 volt, so 16 volt is fed into the Hypex regulators (ignoring the voltage drop across the rectifiers).

The way I had understood the circuit, the "shutdown protection" part, incl. zener D5 is fed by +VOP/-VOP, which is the output of the Hypex regulators. +VOP/-VOP would be +12/-12 volt, and the zener and transistor are driven by that.

Am I missing something? Sorry if this set-up was not clear enough from the start. Reason by the way to use +/-16v instead of plain AC is that I am re-using a transformer that gives too high a voltage, so I am regulating it down before feeding into J6.
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Old 11th February 2019, 08:22 PM   #762
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by maartn View Post
Hi Hans, not sure if I understand that. In my current set-up J6 is connected to +/-16 volt, so 16 volt is fed into the Hypex regulators (ignoring the voltage drop across the rectifiers).

The way I had understood the circuit, the "shutdown protection" part, incl. zener D5 is fed by +VOP/-VOP, which is the output of the Hypex regulators. +VOP/-VOP would be +12/-12 volt, and the zener and transistor are driven by that.

Am I missing something? Sorry if this set-up was not clear enough from the start. Reason by the way to use +/-16v instead of plain AC is that I am re-using a transformer that gives too high a voltage, so I am regulating it down before feeding into J6.
O.K., I thought your scope image was showing +VOP.
Then the next thing to try would be to change R54 into a much lower value, like 680 Ohm or 820 Ohm, thereby driving T3 less into saturation.
Because switch off should happen within 4 msec and that's not what I see.


Hans
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Old 12th February 2019, 06:21 AM   #763
maartn is offline maartn  Netherlands
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Thanks Hans, then that is the direction I will look in. Out of interest: how did you calculate the 4 msec?
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Old 12th February 2019, 04:32 PM   #764
Hans Polak is online now Hans Polak  Netherlands
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Originally Posted by maartn View Post
Thanks Hans, then that is the direction I will look in. Out of interest: how did you calculate the 4 msec?
That is the relayís switching time.

Hans
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Old 13th February 2019, 07:16 PM   #765
maartn is offline maartn  Netherlands
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Measured the voltages:
+VOP = +12,20 (output of regulator)
-VOP = -11,90 (output of regulator)
Vbase = -11,15
Vcollector = -11,85
the voltage between the zener and R24 is -5,60

That would mean that the base current is around 2 mA as it should be... Details on the parts: T3 is a Taiwan Semiconductor BC846B and D5 is an NXP BZX384-C18.

Last edited by maartn; 13th February 2019 at 07:19 PM.
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Old 13th February 2019, 07:30 PM   #766
Hans Polak is online now Hans Polak  Netherlands
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The relay draws some 4mA, so the transistor having a Hfe much higher than 100, would need at the most 0.4mA instead of 2mA.
Thatís why I advised to try a much lower value for R54, to drive the transistor less deep into saturation.

Hans
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Old 18th February 2019, 08:08 PM   #767
maartn is offline maartn  Netherlands
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Thanks Hans, I am learning a lot here. The relay I used is TE MT2-C93402, which draws 12 mA, so I wanted to use 560 Ohms. That didn't solve it. I went as low as 390. Still "pop", but the discharge curve did change (see attached, measuring supply to the onboard regulator this time). Then I mounted a small pot in place of the resistor and found the minimal resistance for saturation: ~260 Ohms. Still "pop"... Then I tried disconnecting the DC supply from the BPBP board (instead of switching off mains like I did before). No more "pop"! The root cause must be somewhere among the mains switch, the transformer and the pre-regulator, but those will be replaced anyway. In the meanwhile I put the 390 Ohms resistor back on, for fast off-switching.
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File Type: jpg 390 Ohm.JPG (167.1 KB, 211 views)
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Old 18th February 2019, 08:39 PM   #768
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by maartn View Post
Thanks Hans, I am learning a lot here. The relay I used is TE MT2-C93402, which draws 12 mA, so I wanted to use 560 Ohms. That didn't solve it. I went as low as 390. Still "pop", but the discharge curve did change (see attached, measuring supply to the onboard regulator this time). Then I mounted a small pot in place of the resistor and found the minimal resistance for saturation: ~260 Ohms. Still "pop"... Then I tried disconnecting the DC supply from the BPBP board (instead of switching off mains like I did before). No more "pop"! The root cause must be somewhere among the mains switch, the transformer and the pre-regulator, but those will be replaced anyway. In the meanwhile I put the 390 Ohms resistor back on, for fast off-switching.
Great, thatís what they call a learning curve 😁

Succes, Hans
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Old 27th February 2019, 08:00 PM   #769
maartn is offline maartn  Netherlands
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Got the preamp in a decent enough state now to plug it in to my main set. See attached pic and the ones on this post.

I was curious to know the output impedance so I measured it and got to 22 Ohms. Anyone else ever measured it and got to the same number?
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File Type: jpg bpbp.jpg (1,007.0 KB, 139 views)
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Old 28th February 2019, 02:24 PM   #770
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by maartn View Post
Got the preamp in a decent enough state now to plug it in to my main set. See attached pic and the ones on this post.

I was curious to know the output impedance so I measured it and got to 22 Ohms. Anyone else ever measured it and got to the same number?
That's Ro connected to the output amp, so the 22 Ohm is exactly as expected when the other side of the measurement is Gnd.
But in the connection to the main amp when using a balanced input also, output impedance is 44 Ohm.


Hans
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