------------------------------ Date: Thu, 16 Sep 93 10:21:21 EST From: peterca@ento.csiro.au Subject: Re: Shunt regulated push-pull design :From: spaaejg :Subject: Shunt regulated push-pull design :Date: Tue, 14 Sep 93 14:19:13 +0100 : :I have some questions concerning the so-called shunt regulated push-pull (SRPP) :circuit configuration. : :1) Can anyone confirm for me that this is the correct configuration for :a triode amplifier stage connected in SRPP? It's one of them. : : | HT : | : --- : : ___ _ _ _ : | : | ___ : | / \ : | | : | R : |________|_______C______ OUTPUT to next stage : | : --- :INPUT :______________ _ _ _ : | : | ___ : | / \ : R ____| : | C R : | | | : ___|______|___|_____ GROUND : :2) If so, is the DC voltage on the anode of the lower valve (i.e. the output :line) exactly half of that on the top one (i.e. the HT)? Yes, it is. :3) Finally, is it worth using this for the input stage of a power amplifier :rather than the usual fixed resistive load? Does the HT regulation :translate into better sound quality? This is what I have been thinking. The distortion from this is so good that no feedback is necessary for this to stand alone as a gain stage. Thus you could perhaps use this to drive an output stage with only local feedback. Alternatively, you could apply feedback via a resistor (say 100 ohm) which sits under the lower cathode resistor and its bypass. I havn't yet tried any of this yet. I use this circuit twice for my phono stage (from an Elector article in about 1986). First comes the circuit with a 12AX7 then passive RIAA equalisation then the same circuit again with a 12AT7. In both cases the cathode resistors are 1K bypassed by 470uF and 0.47uF. In my case this privides enough gain to drive my power amp direct (via source selection and a volume control). Originally I had a further gain stage (following the Elector design) as a line amp which gave and extra 22 db and lower output impedance. This used a 12AU7 and 1K5 cathode resistors. I found I didn't need the extra gain and this stage contributed more noise than the phono stage (which is very quiet). I also bought a valve moving coil step up amp which used a similar circuit (now passed over in favour of a good transformer). This used 12AX7s and differed from the above by haveing a 2K resistor for the upper cathode and the lower cathode connected to ground. There was a coupling cap on the lower grid which meant the lower grid is free to float where it likes. I now use this circuit with a 12AT7 as a gain stage/buffer for my modified CD player. When I removed the line (12AU7) amp I was surprised to find that the sound of the CD increased by a larger margin than the phono. Thus I wondered whether there was an advantage to the version of the circuit used for the CD. I like the fact that it eliminates the large electrolytic cathode resistor bypass and replaces it with a small film cap on the lower grid but it doesn't have the symetry of the Elector circuit. I have yet to try this modification with the phono stage. What I have tried is to remove the capacitor bypass for the phono stage. This I thought might improve the sound due to local feedback but with a penalty of reduced gain. In fact I though it sounded marginally worse with about 3db loss of gain per stage. A third variant on this circuit (from Glass Audio) uses a coupling cap to feed the upper grid form the lower anode and has a larger plate resistor for the lower triode inserted between the upper cathode bias resitor and the lower anode. The lower cathode resistor is bypassed but this apparently only gave an increase in gain of 1/2db. I havn't tried this circuit and am put of by the need for an extra coupling cap. Peter Campbell (peterca@ento.csiro.au) C/O Div. Entomology, C.S.I.R.O. Box 1700 Canberra A.C.T. 2601 Ph.61-6-2464158 (w), 61-6-2516213 (h) Australia 61-6-2464173 (fax) ------------------------------ From: dunker@Lise.Unit.NO (Thomas Dunker) Subject: Re: SRPP circuit comments Date: 4 Mar 1994 13:57:23 GMT Thank you for the additional info, Peter. I didn't mean to imply that you CAN NOT use both halves of a twin triode in the same amp. Rather, it is a good idea to try to stay safely within the limits of the tube to ensure dependability. Your own solution seems to be a good compromise, and could probably be applied to most of the actual tubes one would use in the SRPP circuit. There probably is some theoretical advantage to using both halves of the same tube in the same amp. You mention shortened signal path, which often seems neglected in much modern PCB-built equipment, and which I think should be a goal in itself. (Why bother with $1000 interconnects if you have several feet of low grade copper PCB tracks in the signal path?) Your thoughts on channel separation and decreased sensitivity towards matching make perfect sense as well. (I've seen examples of tube preamps with both channels using the same twin triodes all the way through. After five tubes, the channel separation was still better than 90 dB, so worry not!) Tom sdsdsdsd