I’m giving it a crack... fully DIY built phono preamp

[neevo]

I’m a competent desolderer and solderer. I’ve spent a lot of time and effort rebuilding a few parts including amps and a couple of preamps based off the Little Bear preamp.

My Little Bear looks beautiful and I’ve had such fun making it, but I long for something quieter and I want the satisfaction of knowing I built it myself. I put a lot of time in to my preamp and even relocated the transformer into a separate chassis but I’m still cursed with hum at a level that’s simply not enough for the resolution of my system, especially with a HOMC and some quieter records where I want to crank the volume a bit. Generally it’s fine, but my system is working so well and sounds so good that “fine” is not good enough anymore.



Rothwell mentioned having a go at point to point wiring in my LB thread so I reached out to him and he was incredibly gracious with his time and knowledge. He mentioned an RCA design (I think that’s what it’s called) from Valve Wizard. They used to make PCB’s but they’re no longer available and to be honest I want to have a crack at the wiring point to point instead.

I’m expecting a huge amount of learning but I’m hoping I can make something reasonable and quiet, especially with some help from the experts here.

I’ve been doing a tonne of reading, checking the schematic and parts list and I think I’m mostly comfortable with it, at least enough to think about how I’d start the process.

I’ll also build a cabinet from scratch too as I’ve done with my friends chassis as I want that level of satisfaction too. 3mm Alu and hardwood ends (the plastic still has to be pulled off the Alu):



If I can get it quiet enough with some general tubes, I’m toying with the idea of buying some really nice Telefunkens

I’ll build the amp with 12AX7’s and an AU7 for the cathode, also based on Rothwells recommendation.

The only part that confuses me is the power supplies. I’m pretty much ok with the HT supply and I’ll hunt down a good quality 300v transformer that I will try and shield with a metal divider in the chassis.

The one that really has me stumped is the heater supply. I’m pretty sure I want to go down the rectified DC path for the heaters to eliminate any potential hum but I’m not clear on the parts required for the DC rectifier and also what sort of transformer is required to properly supply the rectifier, 2x AX7’s and an AU7.

 

[gadget73]

The RCA phono preamp is not bad, and not overly complex. You can find the schematic and parts list in the back of most of the RCA tube manuals.

I would suggest adding a cathode follower or an op-amp buffer to the output though.

 

[neevo]

The schematic has a cathode follower

 

[rothwellaudio]

I may have mentioned this to the OP in a private message but I'll say it again on this thread for the benefit of all:
There are basically two types of noise that can plague a phonostage - hiss and hum.

1) Hiss will be determined mostly by the first tube in the circuit and there's not a lot you can do to reduce it with fancy components etc. If you have a few tubes (of the same type) to try, you may find some are a bit quieter than others. Use the quietest at the input of the circuit and the noisier ones for the second stage or output buffer. Generally, metal film resistors will give you a bit less hiss than carbon film or carbon composition resistors. Probably the most important resistor for hiss will be the first anode load resistor. If you want to buy any fancy/expensive resistors, use them as the first anode load resistor.

2) Hum will probably dominate the hiss and will be produced either by HT power supply ripple, the heater supply, the proximity of the mains transformer or a poor wiring layout.

A voltage regulator for the HT should be capable of dealing with the HT ripple but needs to be able to work at high voltages. Simple RC filtering may be good enough if you don't want to bother with a regulator. It's easier but probably not as good as using a regulator.

The heater supply could be simple AC, but a DC supply would be better. However, there's probably no advantage to a DC supply unless it is regulated. Fortunately the heater supply is at a low voltage so an off-the-shelf regulator such as an LM317 is a simple solution. Bear in mind though that the current drain for the heaters is quite high and the regulators need at least 3V of headroom. It might make sense to run the heaters in series at 12.6V rather than in parallel at 6.3V. The regulators probably need to be on a heatsink too, though bolting them to the case will be enough (most likely).

The proximity of mains transformers won't be a problem if they're housed in a separate enclosure which is far enough away from the audio circuit.

The wiring layout can make a big difference but it isn't obvious from looking at a circuit diagram what the best layout should be. The advantage of a tag strip layout is that it's easier to experiment and make changes than it would be if everything was on a PCB. With a PCB you're at the mercy of whoever laid out the board and if you want to make changes it's a proper pain in the backside to start cutting tracks and inserting link wires.
Some background reading on laying out a circuit may help here.

Hope that's of some help

 

[Tubeglowpio]

I have a little bear t10 and never had noise issues with it on 96db speakers. A tube phono is something I have not built yet as well, I found an upgraded rca schematic somewhere a while ago, had a little more gain and was optimized I guess.

 

[kward]

There are quite a few well traveled paths on the venerable RCA phono preamp. For example these threads come to mind (in no particular order):

http://audiokarma.org/forums/index.php?threads/whos-built-this-phono-stage.642448/
http://audiokarma.org/forums/index.php?threads/rca-phono-preamp-2.731999/
http://audiokarma.org/forums/index.php?threads/rca-riaa-tube-phono-stage-using-7025-12ax7.275753/
http://audiokarma.org/forums/index.php?threads/my-take-on-the-rca-phono-preamp.656898/

I don't have the RCA tube manual in front of me, but I am pretty sure the "stock" circuit did not include an output buffer stage. I would recommend adding one if you are going to use a long cable run to your preamp or integrated amp, or that amp or preamp has lower input impedance. I think a 12AU7 would work just dandy in that role. Some people shun 12AU7s because of their non-linearity, but in this role as a cathode follower on a phono stage, it won't matter a bit. Nearly any good preamp tube could be used as the output buffer in this application such as 6FQ7, 6DJ8, 12AX7, 12BH7, etc. You could even add a FET (Field Effect Transistor) buffer. Several guys over at DiyAudio have done that.

My recommended must haves for this build would be:
1. Regulated high voltage supply using shielded multi-wind power transformer that delivers both high voltage and filament voltage outputs
2. DC filament supply using a three terminal regulator on a heat sink. Regulation on a filament supply isn't necessary, but still, it's the easiest and fastest way to do it, in my opinion.
3. Decent layout and good point-to-point wiring skills

 

[Lavane]

FETs are used in one of my tube phono stages. The tubes used are 12ay7's.

 

[BinaryMike]

1) Hiss will be determined mostly by the first tube in the circuit and there's not a lot you can do to reduce it with fancy components etc. If you have a few tubes (of the same type) to try, you may find some are a bit quieter than others. Use the quietest at the input of the circuit and the noisier ones for the second stage or output buffer. Generally, metal film resistors will give you a bit less hiss than carbon film or carbon composition resistors. Probably the most important resistor for hiss will be the first anode load resistor.

I believe it was Merlin Blencowe who confirmed that hiss can be reduced by lowering heater voltage, but only when anode current is below 1mA. You will see this feature in numerous RIAA preamps from the sixties and earlier.

 

[analog addict]

I'm the auther of the "Who's built this phono stage?" and I apologize for it being essentially useless due to half the pix being held for ransom by Photobucket. However, that being said, I also have some advice for you.

Use a SS bridge and a 12 VDC voltage regulator chip. Helps keep the hum out of the heater supply.
Reduce the ripple on the PS voltage to as low as you can get it.
Use shields on your tubes
Only add a cathode follower if you anticipate using the pre with SS gear. I only use this pre with toob gear, so I didn't need it.

If you'd like the schematic, I'll see if I can dig it up for you....

 

[neevo]

@kward with the power supply. Do you recommend a buy a dual wound transformer to run the HT and heaters? Are there any recommendations for reasonably priced good quality units?

Also with the regulated DC supply, this I assume would be run off a 6.3v secondary winding (if the above is correct). Can you help me understand if this regulated supply is an off the shelf item I can buy or would need to make from parts. I’d be comfortable with both but I need a bit of guidance.

@rothwellaudio i looked up the LM317 and it looks like part of the DC regulator (dc to dc regulator), so again I’d assume I need a few more bits to build the AC to DC converter and the add the regulation. Is there a resource to point me in the right direction here? Also I assume the 6.3v series vs 12.6v parallel would also change my requirements.

Apologies for the simple questions. I’m still trying to learn here.

@Analogue addict can you explain your recommendations a little more? I have no idea what a solid state bridge is to be honest so I’ll do some reading. Is it a simple device to put in for the heater supply? I’m keen to go DC for heaters if possible to reduce potential hum, but I’ll need some guidance to help build up my understanding of the theory and parts required to actually build it. Also I have no idea how to ensure “ripple” is kept to a min, so I’ll do more reading here but any help to understand it also appreciated.

Obviously this is not entry level stuff and I’m out of my depth already. So more reading to do!

 

[neevo]

@rothwellaudio in relation to the first anode resistor, I assume that’s R3:



I’d assume Metal film resistor here would be the best option if I wanted something quality as you mentioned. What about wire wound or is that not beneficial?

 

[Lavane]

Your T-10 should have a bridge rectifier for the heaters. Four diodes or a one piece bridge rectifier.

 

[analog addict]

schematic. Add a 1Meg resistor to ground after the last 0.22 uF cap before the output.

Untitled by Analog Addict, on Flickr

The DC heater supply. This made use of two 6.3V winding on the PT. You could buy one with a 12.6 winding instead.

Untitled by Analog Addict, on Flickr

This is the 12 V regulator

https://www.radioshack.com/products/radioshack-12v-fixed-voltage-regulator-7812

And I believe this is the bridge....

https://www.radioshack.com/products/50v-bridge-rectifier

Ripple is the amount of AC variation left in the DC voltage produced at the output of a filtered power supply. This can be modeled using PSUD

http://www.duncanamps.com/psud2/index.html

However, I would caution you that a phono pre amp is not an easy first scratch build.

 

[BinaryMike]

Do you recommend a buy a dual wound transformer to run the HT and heaters? Are there any recommendations for reasonably priced good quality units?

Antek is a good source for economical dual-primary toroids with inter-winding (Faraday) shields.

 

[kward]

@kward with the power supply. Do you recommend a buy a dual wound transformer to run the HT and heaters? Are there any recommendations for reasonably priced good quality units?

Yes, because you won't get better performance with using separate high voltage and low voltage transformers. Getting a custom wound EI type to your exact voltage needs with external Faraday shield is not that expensive from Heyboer transformers, although shipping to Australia might be a concern.

A power transformer with a Faraday shield isn't strictly necessary if you are skilled at layout and building. But for the lesser skilled, I think it will greatly assist in getting a quiet phono preamp.

Also with the regulated DC supply, this I assume would be run off a 6.3v secondary winding (if the above is correct). Can you help me understand if this regulated supply is an off the shelf item I can buy or would need to make from parts. I’d be comfortable with both but I need a bit of guidance.

If you use 12V filament tubes, I'd use a 12V three terminal regulator. Like the one AA posted above, or here's a schematic of one I built, as well as a picture of the actual working unit:



For your build, replace those two series wired 6U8 filaments with two 12AX7 filaments wired in parallel. I used a 2940 three terminal regulator because they offer a low dropout voltage, but other types would probably work just as well. For example, Analog Addict depicted essentially the same circuit but with a 7812 regulator.

Front:


Back:

 

[BinaryMike]

I used a 2940 three terminal regulator because they offer a low dropout voltage, but other types would probably work just as well. For example, Analog Addict depicted essentially the same circuit but with a 7812 regulator.

I wouldn't attempt this with a 7812 because its dropout voltage and tolerance brings it perilously close to ripple troughs in the rectified 12.6VAC. Allow for a little drop in AC line voltage and your regulation is kaput.

 

[Mike Stehr]

I repurposed a Webster Electric RTR preamplifier head unit for the RCA passive phono stage. I made the circuit on a bread-board using 6SL7, and decided to build a finished unit...or repurposed...
This time I'm using 12XA7 because the Webster uses 9 and 7 pin sockets. I think the RCA circuit is designed with 7025 in mind, which has gain/amp factor of 70, but maybe mostly as a sales pitch.

I dunno...I gutted the Webster and am using the original PS transformer, 6X4 rectifier, with the stock CRC supply. I did raise the 15K resistor to 68K, to drop a bit of voltage to get the 12AX7/7025 plate voltages within spec. The decoupling caps for 12AX7/7025 plates were raised from 20uF to 50uF. I replaced the old cap cans with what I needed for JJ/Tesla. Cathode bypass caps for 12AX7/7025 are 35 volt 100uF

I'm heating with AC, with the 6.3 volt winding's CT to ground. I made an inverse RIAA network, (the old colony design?) so I could monitor square/sine wave with the phono pre-amplifier circuit.
I used a 600 ohm output signal generator through the IRN into the phono stage with 1 volt. I didn't load the output, I just put the scope probe there without the load.

Sine-waves looked pretty good I would think, square-waves look okay until around 10kHz, then the top and bottom left edges start rounding over and continuing as frequency rises.
There is some low frequency noise I could see on the scope, 60-120Hz...I'm thinking 60Hz...it's in the upper micro-volt region...

I went ahead and clip-leaded a junk speaker, and then a horn to the output of the phono stage with the signal gen (1 volt out) connected to the IRN, then to the phono pre-amp. Listening for hiss/hash or hum, I couldn't really hear anything even with the volume pots cranked. I know that's probably a redundant thing to do compared to a sensitive cartridge amplified through an amplifier, but I figured it may help detect noises.

I should measure over things again with a AC VTVM, but I'll give the unit a shot and see if it's listenable, and what to do next. The K&S Musicmaster Point One Five is re-lubed and ready with the original idler wheel swapped back in. The pitch on the TT is steady as a rock once dialed-in...

Sorry...it seems I kinda jacked your thread. There may be something to glom from it.

Mike

 

[neevo]

Looking at transformers I’m a little confused.

http://www.antekinc.com/as-3216-300va-16v-transformer/

Does this transformer have a primary and secondary winding?

I can find a Edcor:

https://www.edcorusa.com/xpwr005

Also can some validate my assumptions? I need a 300v primary winding and I’d assume a 16v secondary capable of providing 500ma with some headroom if running 3 tubes in parallel with the 3v drop from the regulated DV supply?

Edit: reading a bit more am I correct in assuming I’d need a centre tapped winding for the heaters at around 16v so I can full wave rectify the AC? Then a standard tap for the 300v?

 

[PakProtector]

A large capacitor-input filter like what has been shown poses a noise risk. They have large current spikes required to charge the caps back up. Even in just linestages, I would stick with an L-C filter, and tune the voltage with some R-C stages. Hammond makes a few that are suitable, and you will *STILL* want some 10,000 uF caps as reservoirs. What gets delivered to the heaters will be nearly ripple free, and it will have none of the high-frequency regulator noise which gets to the cathodes through the h-k capacitance.
cheers,
Douglas

 

[kward]

If you use a three terminal regulator to develop your filtered DC for the filaments, you will want to use a 12.6V secondary. A larger voltage secondary than that will require that you spill off the excess voltage as heat in the regulator itself. Instead, if you want to go with L/C and/or R/C filtering, you will need a bit more secondary voltage to play with, say 14VAC or 16VAC, depending on how many filter sections you add, so your end result will be down where you need it at 12VDC after it's filtered.

As an example, in the case of the three terminal regulator approach, you can use either a 12.6V secondary with a bridge rectifier configuration, or a 12.6-0-12.6 secondary with a FWCT configuration, where CT is grounded. Both methods are equivalent as far as the DC output they deliver. Note that each side of the FWCT secondary will pull half the current of the secondary used in the bridge approach.