First pre phono design [newbie]

[euro21]

Maybe something based on a ECC88 that is not expensive and is easy to find.

Like this?

 

[max1979]

Like this?

Yes, like that.

I'm trying to understand what solution is supposed be the best as output stage in a pre phono, assuming a u-follower as input stage and considering I'd like to have as much gain and linearity as possibile. Another u-follower?

 

[SY]

Well, you'd probably want the output to be buffered. So the stage driving the buffer doesn't need to have a particularly low output impedance- in my designs, I tend to use a CCS-loaded common-cathode amp.

 

[max1979]

Now that we have investigated the feasible configurations for input-output of a phono preamp, I need an advice about the tube. If I really want to reach 1v-2V as output when the input is 5mV, my phono stage must have a voltage gain of 46-52dB.

Now the ECC88 has a u = 33, so even if I assume I can get the most from the tube using it in a u-follower (or beta-follower) config, I can reach a gain of 20*log(33) = 30dB.
Even if I use two consecutive u-follower stages I'll get 60dB as total gain, but I have to consider the RIAA section that means -20dB, for a real total gain of 40dB... does it mean I have to find another tube? adding another stage?

 

[SY]

Here's a reason why phono stage design is so challenging- that 5mV does NOT represent a max level in the same way that "2V at 0dBFS" does for a DAC or CD player. There actually is no real standard here. So we have to think carefully about what the gain structure of the system is.

If you determine that you want more gain, the place to put it is in the second voltage amp, e.g., use a higher mu tube. Care to guess why?

 

[Merlinb]

for a real total gain of 40dB... does it mean I have to find another tube? adding another stage?

Two ECC83s would manage enough gain. With other valves you'll probably need three stages of gain.

 

[SY]

ECC83 in the first stage can be a problem.

 

[max1979]

Two ECC83s would manage enough gain. With other valves you'll probably need three stages of gain.

Yes, indeed I used ECC81/83 in my first design, then I changed my mind at least for the input stage - now I'm going with a beta-follower.

@SY: for sure now I'm realizing how much work it needs to design a good pre phono but I'm not going to design the most general purpose phono, just something that sounds good (very good ) with my cartridge Ortofon 2m red (Ortofon - 2M Red). But as you said it's a challenge also in this case, for instance I still don't understand exactly what they mean when they say "Output voltage at 1000 Hz, 5cm/sec. = 5,5 mV": are they talking about Vpeak, Vrms, Vpk-pk? Maybe it's a silly question but it could have an impact on the gain I'd like to reach.

 

[SY]

No, that's a very pertinent question. It's RMS voltage.

 

[euro21]

If you use in the second stage cathode decoupling capacitor, or -better- two series LEDs and CCS as anode load, gain is about 50dB.... but at higher gain noise of first stage (hiss and hum) may be confusing.

Two stage phono is simple, but without output buffer (probably CF) -as SY wrote- output impedance is not so low (10-20kOhm).

 

[max1979]

Ok, here my second attempt.

Beta-follower (ECC88) followed by a common cathode (ECC83).
For sure this config can be improved (better RIAA response, active load in the output stage), but at least I can reach 46dB according to micro-cap simulation.

A part from this specific schema, I have found something interesting. The output stage has been copied from Mullard ECC83 datasheet http://www.4tubes.com/DATASHEETS/SCANS-Original/E/ECC83-Mullard-Datasheet-Page4.gif as it was, in order to have a valid (and well known) starting point.
According the datasheet this config should provide a Vout/Vin = 60 and it's almost what I can see during the simulation - I see more than 110 because I'm considering the voltage between the "two outputs". What is more useful, the datasheet provides also a distortion value approximately proportional to the voltage that in my case should be close to 0.1-0.2%

 

[DF96]

Perhaps a point of confusion is that the cartridge sensitivity may be well-defined, but the recorded velocity is not. There is not a hard limit, so it is unlike amplifier clipping or digital full-scale. You need enough gain to cope with LPs cut quietly, and enough headroom to cope with LPs pushing the boundaries. In this respect a phono preamp is a bit like a microphone preamp.

 

[SY]

Ok, here my second attempt.

Question- why do you have a balanced output but a single-ended input?

Second question- why no buffering of the output?

I have more questions...

 

[rayma]

Perhaps a point of confusion is that the cartridge sensitivity may be well-defined, but the recorded velocity is not.
There is not a hard limit, so it is unlike amplifier clipping or digital full-scale. You need enough gain to cope with LPs
cut quietly, and enough headroom to cope with LPs pushing the boundaries. In this respect a phono preamp is a bit
like a microphone preamp.

See the section "Design Considerations of the Vl5 Type IV Phonograph Cartridge" in this link.
High Fidelity Phonograph Cartridge - Technical Seminar | Shure Technical FAQ

 

[max1979]

Question- why do you have a balanced output but a single-ended input?

Well... maybe I'm wrong here.
If I have to add a buffered output (the line stage) it's more convenient to add a single-ended stage. But I need to put the two half of the ECC83 in serial if I want to keep the 100x factor. Morgan Jones' book could help again, if I well remember he designed a common cathode based on ECC83 somewhere in the book.
A balanced output would be a good choice for the finale stage (I need to feed a final amp with balanced input) but I have to keep things easy here.

Second question- why no buffering of the output?

This pre phono is going to be part of a pre ampli that will include other inputs. So I'll design the buffer (final stage) later on - (ECC82 based?). I need to add:
- a volume control
- a selector to switch from one input to another (I assume relais based)
- the buffered output (with a very low gain?)
The preamp will feed a final amp based on Hypex UcD400 so I calculated a 2V-5V is needed to get the max from the class-D amp

 

[SY]

For buffering the output, you might consider MOSFETs. Morgan and I did some measurements on source followers vs cathode followers under different loading, and the source follower was better every time. You'd want low Crss.

 

[max1979]

Ok, redesigned with single-ended.
Considering two ECC83 give too much gain, I reintroduced ECC88 hoping this could help the bandwidth if I have well understood Morgan advices related to Miller cap.

Now voltage gain seems to be too much so I added a 100k pot between the two anode followers so I can control the gain between 48 and 54dB (added a css too). I'm not sure how to obtain a valid indication of the distortion, I just run FFT to measure the difference between 1KHz signal and the firt harmonic (~58dB) but I don't know if it has any meaning... I guess a measure in the real world is the only way

 

[SY]

What is the function of the CCS there?

If you have to put in divider networks to get the gain down, that's a flashing light that you may want to reconsider some aspects of your design. I'll politely not bring up the output impedance.

 

[max1979]

What is the function of the CCS there?

Maybe I misunderstood your advice, you said "I tend to use a CCS-loaded common-cathode amp" so I went for a common cathode with CSS

If you have to put in divider networks to get the gain down, that's a flashing light that you may want to reconsider some aspects of your design. I'll politely not bring up the output impedance.

I agree: I don't want to throw away gain. The problem here is that I need a bit more gain than what I can reach with two stages: let me explain better what I see (maybe there is something wrong in my point of view).

We have 3 stages: input stage, RIAA stage, output stage.
Running a simulation I can see the input stage give me a gain of 30dB, the second stage (RIAA network) give me an attenuation of 25dB and the last stage give me a gain of 34dB. So the final gain of the pre phono is a "standard" 39-40dB that is lower than needed (46-50dB) - I'm missing 5-10dB.

Please consider I have reviewed the schema (in attach now) using ECC83 in the output stage to have a high u tube. Even a SRPP based on ECC83 seems to not change the scenario. So the idea was to introduce another voltage gain stage. Obviously in this case I need to control the gain because, a part from the tube used, I'll get much more than just 5-10dB.

 

[SY]

By "CCS-loaded," I meant the anode load. Sorry if I was unclear. Your output impedance here is still cripplingly high.

I don't know if it will be helpful or not, but you might take a glance through the "His Master's Noise" writeup over in the Articles section. It's not exactly what you're trying to do, but there may be some ideas in there that you could use.