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Old 27th February 2006, 09:02 PM   #1
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Default Hybrid amplifier.. Please help..

Hey everyone,
I posted about this in the Tube section a while ago but i thought i should post it here now because it is related to the SS section of the amp.
I am in the process of putting together the first prototype of the following on a breadboard, any reasons why it wont work? anything i should change before i power it up?
I know i may have problems with DC offset but there will be an output capacitor in it for testing and i'll sort that out later after i know it works or not.
Anything else i have missed?
Power supply will be +/- 30V
MAny thanks,
Owen
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Old 27th February 2006, 09:41 PM   #2
AKSA is offline AKSA  Australia
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Place a quality cap of 100nF across R3.

Use a quality electo across the two inputs so you have one input point. I suggest a 22uF 6.3VW NX Black Gate for best results.

Hugh
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Old 27th February 2006, 10:16 PM   #3
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i was told i cant use one cap accross the two inputs and i have to use one cap on each input and then tie them together int he other thread?
But it sounds like a plan.
Owen
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Old 28th February 2006, 01:09 AM   #4
ilimzn is offline ilimzn  Croatia
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Your R5, VR1 and R8 need to be proportionally bigger. As they are now, they will be passing most of the current instead of the BD139 used as the bias servo. This will make the BD139 less effective than it should be.
Try R5=10k, VR1=4.7k, R8=2k7. You have approximately 4x transistor Vbe from one input to the other. This is the bias voltage of your output stage. The circuit with the BD139 is called a Vbe multiplier, and it multiplies the Vbe of the BD139 by 1+(R5/(VR1+R8)). For the lowest setting of VR1 (=0 ohms), look for R5/R8 to be slightly more than 3, altogether this will guve you a bit more than 4x Vbe for maximum bias, and still you will be able to get bias to zero with the VR1, for initial testing.
You can use one cap across inputs IN ADDITION to the two caps that couple the SS part to your tube front end. In most designs the third cap is not used as the two input caps essentially double up as the third one, just of a much lower value, sometimes too low. In these cases, the third cap like AKSA mentiones may prove useful.
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Old 28th February 2006, 02:13 AM   #5
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so if i use say.. a 2.2uF polyprop cap on each from the tube stage to each of the inputs and have a 22uF non polar electrolytic cap bypassed with a o.1uF poly cap accross the two inputs this would be a better way of doing it? Thanks for the advice on the resistors, i will try what you suggested, i take it to get the bias to 0 i have to have VR1 set to its maximum?
this is my first real jump into SS, i have built quite a few tube circuits before and have a HND in electronics engineering, i just am not really familiar with SS audio circuits yet.. but i have learnt a lot in the last few weeks..
Many thanks,
Owen
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Old 28th February 2006, 02:19 AM   #6
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like this i mean
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Old 28th February 2006, 09:05 AM   #7
sajti is offline sajti  Hungary
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Quote:
Originally posted by DoomPixie
like this i mean
Hi,

it doesn't work without DC servo. There will be large DC offset on the output.

Sajti
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Old 28th February 2006, 10:10 AM   #8
ilimzn is offline ilimzn  Croatia
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Your C1 is not necessary if you already have C4, and C5 is not necessary if you have your two 'IN' points connected together before the 2.2uF caps (like they should be), as the two 2.2u caps in series are already in parallel with C4.
You also need about 100n in parallel with R3, like AKSA suggested.
Ultimately, Sajti is correct, it will not work without making a potentially huge DC error on the output, which will make it behave completely differently than it's supposed to, assuming it does not, by some miracle, stick to one rail. I'm sorry if this is blunt, but for someone with a HND in electronics engineering, I find it a bit odd that in this thread, people have been correcting errors that reflect unfamiliarity with basic electrical rules.
Without a load, your circuit will saturate to one rail, because of current source inequality (that would be your R4, R7, D1, D2, Q6 and presumeably Q7). You have two current sources connected in series - the voltage at the connecting node is undefined for the ideal current source, unless there is an impedance from that node to a voltage rail or ground, to close the current loop and define the voltage by the drop across it, with the difference of currents of the current sources, passing through it.
For real current sources, which are non-ideal, the connection node will be close to either of the rails. The circuits around it are a voltage follower, so the output voltage will, as the name suggests, follow - without a load, or with an AC coupled load, it will stick to one rail. With a load, one uA of input current difference, will easily make about 10mA of output current difference, i.e. 80mV across 8 ohms of load. Except that you will be VERY lucky if your current sources are that close, count on 100s of times the error above.
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Old 28th February 2006, 11:26 AM   #9
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Keep it simple.
Why not get rid of the current sources altogether and replace them both with 10k resistors? This will keep the offset manageable and will be fine provided the driving stage can drive 5k ok.
I would advise against a cap across R3.
With the 2.2uF polys connected together you really don't need C4 or C5 and a half-decent 10uF for C1 as you had will be just dandy (don't guild the lilly with exotic capacitors at this stage) - your silicon is not worthy.

As you are new to silicon be aware of a few things. Transistors are extremely non-linear and have much reduced bandwidths compared with valves. Driving them requires much consideration as they have realtively low and non-linear impedances. Complimentary pairs are usually not, at least not enough for audio.
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Old 28th February 2006, 01:00 PM   #10
sajti is offline sajti  Hungary
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Quote:
Originally posted by traderbam
Keep it simple.
Why not get rid of the current sources altogether and replace them both with 10k resistors? This will keep the offset manageable and will be fine provided the driving stage can drive 5k ok.
I would advise against a cap across R3.
With the 2.2uF polys connected together you really don't need C4 or C5 and a half-decent 10uF for C1 as you had will be just dandy (don't guild the lilly with exotic capacitors at this stage) - your silicon is not worthy.

As you are new to silicon be aware of a few things. Transistors are extremely non-linear and have much reduced bandwidths compared with valves. Driving them requires much consideration as they have realtively low and non-linear impedances. Complimentary pairs are usually not, at least not enough for audio.

And if You use 4x5kohms instead of 10kohms, You can do bootstrapping from the output to the common points of the resistor pairs, to get very high input impedance without complicated DC servo circuit.

Sajti
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