B1 Buffer Preamp

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#3,781
aha thanks, well used an online calculator had a 400ohm resistor with the 18v Zener and worked for a while, power slowly rising on turn on up to 17v for the B1 then after a while i guess the Zeener just died as the voltage slowly dropped then nothing !
Hmmm back to the drawing board how to achieve 18v from 26vDC

I think i just fried a Zeener

Perhaps Ill just change the Caps in the B1 for higher voltage and run it from the 26v DC, If 26 is not too high for B1
 
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#3,784
Hey, Thankyou

Decisions Decisions 🙂 upgrading the power caps to run the B1 from 26v seems easiest solution ill Probably go with that. yes Id had a 470ohm resistor.

Does anyone know of a good soft start circuit that wont take weeks to arrive in the Uk lol there's a turn on thump on the B1 which id prefer to get rid of!

im thinking Just get a soft start for the whole amp
 
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#3,786
Because the B1 is a single supply, there will always be a turn on thump due to the output coupling cap charging up, there are a couple of ways around the problem,
1. use a muting relay with a turn on delay to short the output to ground for a few seconds.
2. use a slow ramping supply as in post 2 of http://www.diyaudio.com/forums/solid-state/25698-help-lm317-337-slow-design.html
3. turn the B1 on 5 seconds before the F5
4. leave the B1 permanently powered up

My choice would be for No 3 or 4
 
#3,787
The B1 doesn't need a soft start. Soft start is aimed at high power amplifiers with KVA of transformers.

Sounds like your FETs are not well matched and you are getting a DC offset. A correctly functioning B1 will exhibit very little turn-on thump. It's always wise to turn on power amp after the pre amp anyway.
 
#3,788
I see your Point, thanks well I noticed just now that when i Turn the B1 off the power Led takes about 30 seconds to dim out. and im using 2x 15,000uf caps.
Don't know if thats normal ?

Thanks for the Info itsme I was thinking along the lines of Regulator Initially.

Katieanddad
I kind of just like the idea of Power rising slowly when the amps turned on but I see your point 🙂
Ill check for DC-offset

Btw, the B1 is built inside the massive F5 chassis so both use the same transformer but different windings!
 
#3,789
When you turn OFF the power, the B1 is pulling so little current that it is largely the LED that is discharging the 30mF of capacitance - Quite Normal.

I hate relays - especially in high quality audio. But you could arrange a pair of relays to (1) apply the B1 output to the F5 after say 5 seconds and (2) to apply a significant load to the B1 power supply to discharge it more quickly and/or remove the B1 output from the F5 at power down.
 
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#3,790
Another thing is my PSU for B1 is 22v but when B1 (load) connected is showing as 33v, as I don't have much knowledge on the subject I don't know why this is.
So which is the real voltage ?
Dc offset
Left = 0.01v
Right = 0.34v (slowly decreases over a minute or two)

Hows that for DC-offset ?
 
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#3,792
KatieandDad said:
When you turn OFF the power, the B1 is pulling so little current that it is largely the LED that is discharging the 30mF of capacitance - Quite Normal.

I hate relays - especially in high quality audio. But you could arrange a pair of relays to (1) apply the B1 output to the F5 after say 5 seconds and (2) to apply a significant load to the B1 power supply to discharge it more quickly and/or remove the B1 output from the F5 at power down.
Click to expand...

Ahh ok I see, Im used to the power of the F5 and so On which will drain the PSU rather fast but of course B1 only draws 0.02A and 15,000uf I suppose would be large capacitance in that respect.

Sounds like a good idea, The relay thing is nice but a bit of a hassle and also wouldn't want to degrade the sound.

I actualy just put a 12v Light bulb in series of PSU to see what it will do, it lights briefly on turn on then dims, probably not a good idea but im experimenting 🙂

Well Im actualy thinking of getting some kind of Pre-amp to replace the B1 as it would give more gain.
 
#3,795
The B1 can share a PSU. But you must get the voltage for the B1 correct.

You must also arrange for the correct sequencing of the power up.
A single polarity supply to an amplifier will always give a varying DC pulse through the DC blocking capacitor at start up. Always !
If your sequencing cannot protect your speakers, then you need correct muting.
 
#3,796
The reason that you are seeing the higher voltage with the B1 than expected is because of its very low power drain. If it's measuring 33V then that is what it will be producing with such a low load. 33V is fine as long as the caps are rated sufficiently. I would consider 36V about the maximum voltage for the B1.
 
#3,798
AndrewT said:
The B1 can share a PSU. But you must get the voltage for the B1 correct.

You must also arrange for the correct sequencing of the power up.
A single polarity supply to an amplifier will always give a varying DC pulse through the DC blocking capacitor at start up. Always !
If your sequencing cannot protect your speakers, then you need correct muting.
Click to expand...

My B1 PSU is separate to the F5 PSU just both use same transformer which has 2 sets of secondarys one lower voltage than the other.

I Think ill go for upgrading the caps and powering the B1 with the 33v.
 
#3,800
Iv'e been looking around for Pre-amps to replace the B1 but not having much luck, i know the burning amp pre-amp is available but I expect its not ok for driving F5t?

Do we know of any other Pre-amps ? other wise Ill try to get this B1 working 🙂

AndrewT said:
This seems broken.
Most properly built B1 have zero Output offset. The DC blocking cap and the leakage to signal ground resistor ensure the output offset is zero.

0.01Vdc = 10mVdc is wrong.
0.34Vdc = 340mVdc is broken.
Click to expand...

When you say broken you mean something on the board may be blown ?
 
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