my RIAA stage. Any comment

[SY]

Oh, and you thought that being helpful would be enough for me to stop pestering you. Naive.

There are whacking great signal voltages leaving the stage before equalisation!

How great is "whacking great"? A typical mm will have an output of roughly a millivolt per cm/sec. Max velocities before mistracking are on the order of 30 cm/sec for the better examples, and those velocities are the exception. So I see a max input of something like 30 mv. With a triode in the first hole, we will expect something on the order of a gain of 40-50, a bit less for the triodes I use, a bit more for tubes like 12AX7. So that looks like a max swing at the plate of something like 3V or less.

Now, one might argue that you can see higher outputs when a cartridge mistracks. But the distortion of the phono preamp becomes a pretty minor issue with that diamond chattering about, as long as the designer has thought about overload recovery or put in a bit of bandwidth limiting to get rid of the ultrasonics.

What have I missed?

The cathode follower still has to swing current into the unpleasant load.

Well, it's not that unpleasant. Do you really need 10 ma? With a 20K minimum load, that corresponds to 200V, which, I grant you, IS a "whacking great" amount.

 

[EC8010]

It's not that you need the full 10mA for audio swing, it's that vertical swings are so non-linear that they have to be made a small proportion of the quiescent current. In order that the noise of later stages doesn't significantly add to that of the first stage, you need quite a healthy level before applying your equalisation (which must introduce a loss of 19dB).

There used to be a rather nice graph of recorded velocities (Shure?), which showed double the velocity you mention. But the killer is dust and scratches which mistrack to produce even higher amplitude signals. If the pre-amplifier overloads on these, it prolongs the effect. To my ears, the main difference between RIAA pre-amplifiers is in the relative level of clicks and scratches.

 

[SY]

Ah, thanks. The rails are getting a bit smoother now, we must be out in a rural area. My idee fixee for a long time has been that any demonstrable audible differences between competently designed amps/preamps/boxes of gain is overload recovery.

 

[EC8010]

My idee fixee for a long time has been that any demonstrable audible differences between competently designed amps/preamps/boxes of gain is overload recovery.

I think you're right. If you look at the number of people using pre-amplifiers with gain between CD players and power amplifiers, you have to conclude that they drive their power amplifiers into overload much of the time. The least tolerant of overload is traditional silicon, followed by valve push-pull with feedback, valve push-pull without feedback, single ended valve.

 

[etalon90]

Sorry for delay in answering.
My computer was down and I format my 2 hard disk...

I still don`t know ahy my bass il weak (not as mutch as you can think)

I exclude plate choke because i already use them in my dac. The bass in my dac is ok.

I do 2 error in my schematic. The Zin is not 100K butk 50K (i use two 100k in parallel) And the running current is not 16mA burt 26 mA.

I include a frequency reponse of the value I simulated.

 

[etalon90]

Now. I try to add a 1.8K after the LCR for adding load as suggested by a member. (don`t remember witch one...)

All Eq became worse. Around 8Db error on curve.

I remember when I designed this RIAA setup I had 2 option.

1-A 0 impedance source driving the LRC(600ohm) and finish by a 600 ohm load. then, next stage. This is why we see a cathode folower driving a stepdown transformer to keep Zout as low as possible.

2- a out of 1.8K (of the Zout of your driver, 1.8K in my case) This drive a 1.8K LCR without a ending load. As you see in my other post, the theorical value are good.


I run a simulation and probe the current in the capacitor (6.8uf of the first stage) . As you see, it draw less current in bass area than in high frequency. So the weak bass don`t came by dre driver saturation or difficulty to drive LCR.

I run a test. I add a serial resistor on my powersupply to see the effect on sound. My battery+a 50ohm in serie. I didn`t notice any change in tonal balance. So adding big capacitor on the B+ should not help the bass.

I`m not an expert but I thinkink remove the 470Uf on the cathode and running the 2 stage in superpath. What will be the effect on local feedback (I don`t know how to calculate local feedback in tube circuit.)


I have another theorie for my weak bass. The value of the inductor are nominal value at nominal current. So a LCR don`t draw nominal current in choke. In this case, the value of my inductor will not be correct. My simulation program don`t take this phemomen in account. That could be my problem... but how to fix that...

Does it make sense?

 

[fdegrove]

Hi,

Does it make sense?

Errrr...Not to me but then I never really understood how you got it so close to RIAA spec in the first place.

From what I've seen done with the 600 Ohm version feeding a sim with inverse RIAA network it can be tweeked to ruler flat response.

Every single person writing about these LCR seem to be very insistant on the importance of proper termination.
So I am left very puzzled indeed.

Cheers et bon courage,

 

[Mike C]

How great is "whacking great"? A typical mm will have an output of roughly a millivolt per cm/sec. Max velocities before mistracking are on the order of 30 cm/sec for the better examples, and those velocities are the exception. So I see a max input of something like 30 mv.

What have I missed?

What you've missed is the RIAA pre emphasis.
We are always thinking of RIAA in terms of equalisation, in terms of
the severe HF roll off that we apply.
But don't forget that an equal HF boost will have been applied to the
signal before cutting the record.

A typical MM cartridge will have a nominal output of about 5mV, quoted
at 1kHz.
But a similar signal level at 20kHz will have been boosted by almost
20dB. What comes off the record at high frequencies can be 20dB
higher than 'nominal'.
And that's why you're about a factor of 10 out ....

You need a further correction. The cartridge figures above are RMS
(well, that's how they are usually quoted).
The 30mV you worked with has already become best part of 300mV RMS;
and that's roughly +/- 420mV peak. When looking into overloads, we
are interested in peak values rather than RMS.
For a touch more headroom, +/- 0.5V looks like a good design spec to me.

In my case I bias the 1st valve at -1.0V so as to allow 0.5V swing
comfortably without grid current.
This represents +/- 20V going into the RIAA stage; in the nicest way,
is that 'whacking great' enough for you?

Thanks are due to Par on Tube DIY Asylum; his comments to my early
thoughts on Isabelle made me look into this a bit further!
It's nice when we can all help each other.

 

[SY]

The RIAA pre-emphasis is, I think, a bit of a red herring. Magnetic cartridges respond to stylus velocity, irrespective of what the following electronics do. The max velocity that normal cartridges can track, even at high frequencies, is still about 30cm/sec (I got that number from several Audio reviews of cartridges and some material Shure included with my test disc). So that corresponds to 30 mV, whether at 1KHz or 20 kHz.

Going from RMS to peak is a 41% change, I'll grant you, so my 30 mV figure could well be 42 mV peak. EC8010 emailed me a reference that studied actual outputs using a cumulative envelope detector- I'll be interested in reading this. On my own, I looked at max swing at the plate of the first tube in my RIAA stage (no feedback loop to confuse things, stage gain is 110) using a scope with some tracking test records as the source. The most I could see was a 3V swing, and that was exceptional, occuring as my poor cartridge tried to negociate the highest level test bands on the Shure disc.