tube phono amp req'd

[Raj1]

Hi,

Does anyone have any links to an easy to build phono amp, using a low amount of parts but still sounding very good. I have 2 isolation trannies capable of supplying just over 230v each, and would like to use them if possible to save on costs (trannies can be expensive). I remain quite open minded, though I would like something with little or no feedback if possible. All replies welcome.

Thanks
Raja

[Raj1]

The phono will be used with a thorens td150 mk2 I have just bought. I will require a phono stage for a MM cartridge.

Thanks
Raja

[Eli Duttman]

Raja,

The "classic" RCA circuit should meet your need. It uses passive RIAA EQ and no NFB. The circuit can be found in an older edition of the RCA Receiving Tube Manual.

IIRC, the RCA circuit uses a 270 V. B+ rail. 1 of your isolation trafos feeding a hybrid bridge made from 2X UF4007s and a 6X4 into a cap. I/P filter will yield 325 V., less losses. You could use a CLCRC filter, but a series regulator will be better.

DC on the heaters is ESSENTIAL in a phono stage. A 6.3 V filament trafo rated for at least 4X the DC load feeding a full wave voltage doubler made from 2X Schottky diodes will provide the "raw" DC. Use 2X 4700 muF. 'lytics in the doubler cap. stack. The "raw" DC feeds a 7812 3 terminal regulator. Follow the regulator with a 10 muF. 'lytic in close physical proximity. "Snub" each 12AX7/ECC83 heater with a parallel combination of a 4.7 muF. 'lytic and a 10 nF. ceramic right at the tube socket.

If you build your phono stage separate from the line stage, a cathode follower buffering the gain blocks is indicated. The current production JJ ECC99 makes an EXCELLENT cathode follower.

[fdegrove]

Hi,

The circuit can be found in an older edition of the RCA Receiving Tube Manual.

I hold a copy of the RCA-28 manual and the "Phonogram" circuit shows a 7025 with a B+ of 250V.
RIAA correction is passive but a CF would be mandatory as minimum load is 220K.

Is that the one being refered to here?
If so, I could draw a diagram for the phono part + a series reg + CF for it.

Cheers,

[Eli Duttman]

Frank,

That's it alright. Is that 250 V. the rail voltage or is it the anode voltage?

Using genuine 7025s, with their hum bucking heaters, it's barely possible that residual hum levels will be tolerable if AC heating is employed. Personally, I would not bother to try the experiment.

The heater current requirements, at 12 V., for 2X 12AX7/ECC83 and an ECC99 are 700 mA. That's within the limit of the 7812. A fairly big heatsink is in order. To avoid excessive capacitance following the regulator IC, snub the 'X7s with 3.3 muF. 'lytic/10 nF. ceramic combinations and the ECC99 with a 4.7 muF. 'lytic/10 nF. ceramic combination.

[Raj1]

Hi Frank,

If you could draw the circuit you mention that would be great please proceed, if that's ok. If you have any other suggestions that would be ok too. I've spent hours looking for a ciruict on the web. the only other one I have found of interest is this one http://www.diyaudio.com/forums/attac...&postid=201049

Although the B+ requirements are too high for a single isolation tranny.
If I had too I would buy them,

One thing I don't like is too many caps in the signal path, but I suppose that's quite unavoidable. In any case just let us know which one to build Frank (I'd rather let someone else decide).....

Thanks
Raja

[Eli Duttman]

Raja,

Caps. are necessary in the RIAA network and at the cathode follower O/P. However, Frank might scheme out DC coupling between the 2nd gain block and the CF. I STINK at "hacking" out DC coupled tube circuits.

Since we are talking about a RCA circuit, let me "plug" their tubes. On this side of the Atlantic, NOS RCA 7058s are cost competetive with current production 'X7s. I can't comment on availability and pricing in the EU. Obviously, it is perfectly OK to use NOS Mullard, Philips, TFK, etc. tubes.

[Raj1]

Originally posted by Eli Duttman
Raja,

Caps. are necessary in the RIAA network and at the cathode follower O/P. However, Frank might scheme out DC coupling between the 2nd gain block and the CF. I STINK at "hacking" out DC coupled tube circuits.

Hi Eli,

If Frank would do that it would be great. I won't pester him too much as I'm sure Frank's got other things to do. I'm hoping to build something I never feel like upgrading!


Thanks
Raja

[anatech]

Hi Raj1,
Another way to greatly reduce hum is to raise the voltage about +30V connected to the center tap of the heater winding. Cap to ground from there. Your +30V can be tapped off the end of your B+ supply where you normally may have a bleeder to ground.
-Chris

[fdegrove]

Hi,

Here's something I found among my files based on what was originally an active RIAA correction.

The powerxformer states 250VAC but 230VAC should work as well.

The 5814A / 12AU7A CF can be replaced with something better.
I had a 12B4A in mind for that as it's a great tube and allows the use of 2 tubes per channel which in turn allows for optimimum channel separation.

A 7058 can be used instead of the 7025 (both are derivatives of the 12AX7A/ECC83) shown.
I didn't doublecheck the pinout for compatiblility.
Either way, low noise versions of the 12AX7 are recommended for a phonostage.

I didn't bother with direct coupling the AX7 to the CF as I really don't see the point of doing so in a BW limited cct in the first place and secondly it leaves more options in case a source other than phono needs to be fed through the preamp.

If someone sees a mistake, please don't hesitate to point it out.

I once built the version with the active RIAA which was pretty good given the "modest" cost.
Yet this circuit really shines with high quality components as far as anything behind the regulator is concerned.
If possible use handpicked caps for all RIAA components, the accuracy of the correction curve depends on it.


Cheers,