[pftrvlr]

Greeting. I am trying to mod/fix Bada Ph12 amp. The Mosfet is dead, and I am not going get rid them totally.

Before the Mosfet is a cap coupling 6sn7 cathode follower. I have tried connection HT600 phone directly to the CF and I get loud enough sound out of it.

I have two 4K-8 output transformers around. I'm thinking to connect the tansformer to CF output cap to form a parafeed output circuit. Is it a bad idea? I've never seen such a schematic.

 

[nikongod]

there are a few amps that use a parafeed transformer after a cathode follower.

if the amp has enough gain, the transformer will obviously help with current delivery for low impedance headphones. if the amp has inadequate gain (which is more likely) the transformer will make it rough to use with much.

this website (owned by a diyer here, i forget his name) shows how to hook up a CHEAP transformer for a parafeed SRPP/SRPP amp, the transformer should be similar for this one. it also allows switching for all sorts of headphones. i have seen the amp, and it sounded pretty sweet.

link

 

[dBel84]

I suppose you have a reason for merely not replacing the mosfet output stage, I read somewhere that this amp was prone to frying mosfets if the tubes were pulled while the caps still retained a charge? It should be possible to modify the circuit to prevent this but someone will need to figure out the schematic. BUT, that is not what you were asking and I am nearing the midnight hour with a mind that is off wandering the universe. Here's something more to read less-pressivo and the recent tube cad. Be interested to know how it works out for you..dB

 

[pftrvlr]

Yeah, I got this blown Mosfet PH12 from a trade. I intended to just replace the dead Mosfet. But after looked at inside the amp, I decided not to bother the Mosfet. It is way too complicated and crowded to my like. I took out all the Mosfet and related parts, that almost accounts for half the total parts. Even without the Mosfet, it drives high Z phone OK. Someone mentions that a 6SN7 CF has a output impedence about 400ohm.

I drew the schematic and did some PSPICE sim. The sim results are very close to the actual measured value. The strange thing it that the cathode follower driver actually has positive bias. I am really suprised that this thing is working at all.

There are some really good feedbacks here about the Bada PH12. Too bad I don't have a good one to listen to.

 

[pftrvlr]

Here is the schematic

 

[pftrvlr]

This shows the positive bias values.