|
|
Post by deepdark on Jan 12, 2015 8:43:31 GMT -6
Hi all
I'm looking to make an EQP1A but with a new amp circuit. The goal is to get rid of the interstage transformer and the feedback circuit from the output transformer. All of this in a way to save some $, simplifying the circuit and made it more diy friendly to use with a variety of transformer. I know Gyraf made a SRPP design. But I would like to make something I could show to local music store, in case they would be interested in buying some copies. Something that isn't that far from the original circuit, but not an integral copy of someone else, too.
So here it is. I looked at a Cascode design and would like to know if it's a good candidate for such an task? Assuming the filtre section will eat up around 20db, it would be nice to get back all of these and even more. Sould I consider 12AX7 or 12AU7? In know a 12AU7 will result in less overall gain but if I can get back my 20db lost, why not?
Attachments:EQP1A CASCODE.pdf (475.23 KB)
|
|
|
|
Post by deepdark on Jan 14, 2015 8:56:01 GMT -6
Ok, it looks like a bad idea and the tube choice isn't suited to be drive with an output transformer. Maybe a SRPP should be a better idea. Or a cascode with a 12au7?? Anyone tried it with transformers?
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Jan 14, 2015 9:27:18 GMT -6
I am absolutely no tube guru by any means, but as far as i know, SRPP is used for tube output stages because it is an easy way to drive 600 Ohms.
Cascode designs as far as i know are used where you really need plenty of gain. Like up to 60dB or more. Like on the input of micpres or phono input or so. And guitar pres. Pretty overdosed for the 20dB you need in this application. Hmm.
And cascode design has poor PSRR, which means you need a *very* quiet psu to keep hum and noise out.
12AU7 could be easier to setup in cascode than 12AX7, but that's just hearsay...dunno. Less gain for sure isn't a problem.
I hope, one of those dedicated tube guys will chime in and can deliver more on this topic, my knowledge regarding tubes is quite limited.
Just a quick response brain storm.
|
|
|
|
Post by deepdark on Jan 14, 2015 12:24:29 GMT -6
Thanks for the reply. I'm looking at 2 topology now. The SRPP and the Mu-follower. I think I should get a try in designing a MU-follower and see what people think about the circuit.  |
|
|
|
Post by svart on Jan 14, 2015 13:14:51 GMT -6
Cascodes aren't typically used for driving things since they have a higher output impedance than other designs, and thus less ability to drive loads.
The cascode is mainly used for higher input impedance and more input to output isolation, with higher gain and bandwidth being a benefit of the reduced Miller effect.
There are endless ways to go about this. Plate followers, cathode followers, SRPP, Cascode followers, CCD, XPP, SE, PP, etc. You could try them all, and then try all the transformers you'd need as well since there are so many to choose from in all kinds of ratios.
I suppose, the question is, why did the designers originally use a coupling transformer? They must have included it for some reason..
|
|
|
|
Post by deepdark on Jan 15, 2015 8:00:11 GMT -6
Thanks Svart. Actually, I redraw the circuit in a mu-follower topology. There is some little thing I need help to understand, such has how to determine the right transformer ratio/impedance. Here is the new circuit and the maths on the other page. I calculated an Zout of around 556ohm. Attachments:mu-follower eqp.pdf (887.13 KB)
|
|
|
|
Post by svart on Jan 15, 2015 9:37:01 GMT -6
Thanks Svart. Actually, I redraw the circuit in a mu-follower topology. There is some little thing I need help to understand, such has how to determine the right transformer ratio/impedance. Here is the new circuit and the maths on the other page. I calculated an Zout of around 556ohm. What is the impedance you need to drive? 600? This matters since the load on the secondary will be reflected back to the output tube. We need to know what that load on the tube will be to select the right transformer. Also keep in mind that impedance is rated at a single frequency. It will be all over the place at different frequencies and even more so with different loads on the secondary. It's more of a best guess than anything. Measurements are needed after the fact. Generally I've heard all kinds of "rules of thumb" for transformers. One of the most prevalent is the "transformer load 10x the output impedance of the tube". I'm not sure it works, but it could be a start. You could simply try a 5K:600 transformer and see how it sounds. In the end, it's all about how it sounds to you. Seems that a lot of older designs needing to drive heavy loads used straight mu-followers with White cathode followers back in the day. Now SRPP(which is just a slightly modified Mu-follower!) has somewhat taken over since it can drive lower loads with fewer parts overall. |
|
|
|
Post by deepdark on Jan 15, 2015 9:52:17 GMT -6
Thanks for the reply. Actually, it will be use in insert soundcard or straight after a preamp, or compressor. So my guess was to imagine the input impedance of the primary at 600ohm, then the secondary...I don'T know. But, I know I want to exit the unit with 600ohm. So there it is. I'm not sure about how to determine the secondary of the input transformer. Since it's what will load the circuit, is a 600:10K will do the tricks? And looking at the output, if the rule of thumb works, so my calculated output impedance is around 556ohm, * 10 = =/- 5 600K. I should try a 10K:600ohm. Am I right?
|
|
|
|
Post by deepdark on Jan 16, 2015 5:03:11 GMT -6
Ok so input transformer Will be 600:600, as the output one. Something like cinemag should do the trick. Now i was asking myself how to evaluate the upper grid by-pass cap. Is there a general roule of thumb, or some maths i could apply?
|
|
