adding a balanced output
- Thread starter pucho812
- Start date
Help Support GroupDIY Audio Forum:
You are not interested in a transformer? Looking at the data sheet of the 5751 it has rather high plate resistance so maybe a step dow output transformer? You have a lot of gain there so a big step down is something you can afford I guess?
Maybe that is not what you are looking for?
Best regards
/John
Maybe that is not what you are looking for?
Best regards
/John
ruffrecords
Well-known member
5751 is just an industrial strength 12AX7 with a lower mu. Typical plate current is 1mA so it is not really any good as an output stage for which you need a higher standing current.
Cheers
Ian
Cheers
Ian
pucho812
Well-known member
Looks like I need more current available to drive a transformer?
Looks like replace the 5751 as well.
Looks like replace the 5751 as well.
pucho812
Well-known member
interesting.... study time. except want to avoid ic's. The 5751 has to go, me thinks
bluebird
Well-known member
Quick and easy way is to just try it with a 12AU7 or 12AT7, hook a 4:1 transformer up to it and sweep it. See if your low end is acceptable. Try and use the biggest film cap you have lying around. 10uf would probably do it.
pucho812
Well-known member
bluebird said:
So like a 10uf after the output pot into a 10k:600....
ruffrecords
Well-known member
kambo said:what effects has "higher standing current " on audio / performance, ( google is not friendly on this topic)
Standing current is just the dc current through the circuit when there is no signal, sometimes called the bais current or the quiescent current.
If you want to be able to output +22dBu (10V rms) into a 600 ohms load, you need to be able to supply 10/0.6 ma = 16.6mA rms into that resistor or nearly 24mA peak.
In a simple single ended class A amplifier with a standing current of 1mA, the output can swing up to 2mA and down to 0mA so the peak current swing is just 1mA. So the transformer would need a 16:1 ratio to drive a 600 ohm load to +22dBu. To do this the tube output voltage swing would have to be 160V rms (16 times 10V rms) which would need an HT supply of over 450V.
The circuit posted is an SRPP which is a true push pull which means its output current can swing twice as far as a single ended stage i.e 2mA peak with a 1mA standing current. An 8:1 transformer would now be needed and the output would only need to swing by 80V rms or 225V peak to peak. That is asking a lot of an SRPP stage.
The answer in both cases is to raise the standing current. If you use a different tube and raise the standing current to 6mA in an SRPP then you can output 12mA peak. A 2:1 transformer is all you need to easily be able to theoretically drive 24mA peak into a 600 ohm load and the tube output swing only needs to be 20V rms which is just over 110V peak to peak.
An alternative is to allow the maximum output to be less or the minimum load to be higher both of which would redcuce the required standing current.
Cheers
Ian
pucho812
Well-known member
ruffrecords said:
Stuff to digest as usual.
bluebird
Well-known member
pucho812 said:
No pot... Hang The 10K:600 directly off C1/C2. Make the C1/C2 combo the 10uF. Make sure the 10uF is rated at 400V. At least 350V.
Like Ian said the 5751 wont supply enough current but a 12AU7 might do it. Check out Jakob's G9 mic pre output stage. Its similar.
rafafredd
Well-known member
Plate to plate feedback???
Check the drawing. Is it really correct?
If you correct the circuit, you can just use a 12au7 in there, with a 2:1 or 3:1 output trans...
Check the drawing. Is it really correct?
If you correct the circuit, you can just use a 12au7 in there, with a 2:1 or 3:1 output trans...
bluebird
Well-known member
rafafredd said:
What do you mean? in the input or output stage? It all looks good to me...
pucho812
Well-known member
thanks for the help and suggestions gentleman. still digesting info.
ruffrecords
Well-known member
bluebird said:
I suspect he means R1 should be decoupled.
Cheers
Ian
No FB there because the 1st plate is decoupled to gnd.rafafredd said:
Obviously not. Second plate also needs to be decoupled. It would work somewhat, though.
