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Lentek Head Amp

Avon

Avon

pfm Member
If I may, I’d like to ask a few questions about the following “functional” diagram of the Peter Baxandall MC head amp. It comes from the following website https://fromvinyltoplastic.com/thor...228s-and-lentek-head-amp-upgrade-and-rebuild/.
figure-3.png

First, a lot of the circuit is dedicated to filtering noise from the battery. However, the battery doesn’t reference earth, so to where is that extracted noise supposed to flow? The capacitors to the right do have a central earth connection I suppose.

The input transistor implementation looks nice, as it requires no biasing. What is the name of this arrangement though? It has no feedback, so what sets its gain?
 
Its a great little Headamp which I enjoyed for many years as an extension to a MM phono stage
I stripped mine out and used better quality components and it made a difference
I think Jez Arless would be best to post as he did some work improving this wonderful little head Amp
Alan
 
The 10u caps to earth are noise filters for the battery. 15k resistor sets transistor operating current at about 600uA. 47u is emitter bypass to 100R, which gives local feedback. 3k3 sets output gain. Gain is a bit less than 30.
 
Thanks PD. I hadn’t realised the significance of the 100R resistor. You seem to imply the gain isn’t exactly 33, so I’ll have to do some simulation. I’m still mystified by the lack of a DC path to the “virtual” 0V.

That circuit interests me, as I could use it as the first stage in an MM phono preamp.
 
Avon said:
Thanks PD. I hadn’t realised the significance of the 100R resistor. You seem to imply the gain isn’t exactly 33, so I’ll have to do some simulation. I’m still mystified by the lack of a DC path to the “virtual” 0V.

That circuit interests me, as I could use it as the first stage in an MM phono preamp.
Click to expand...

It is heavily compromised to run off a single PP9. If you have mains power you do MUCH better.

Explaining the gain, the transistor run at just over 0.5mA each, so their transconductance is about 1/40 mho (inverse ohms). They are effectively in parallel, so the gain is like what you would get from high transconductance devices and roughly a 120 Ohms resistor, rather than the physical 100R.

There are masses of electrolytic caps in the signal path, everything gets strange at low frequencies, and the noise performance is nothing special, in fact fairly poor. Even if you ignore the transistor noise, the 100R alone gives you 1.3nV/ rt Hz. Good designs will do 6 to 10dB better than that.

I don't know the transistors suggested, but for an MC stage you want something with a very low value of rbb, the base spreading resistance, made in a clean process without too much 1/f noise. This takes you in the direction of medium signal devices. The much loved BC550/BC560 are a bad choice, with rbb of well over a hundred ohms. BD139/BD140 have terrible 1/f noise. Ztx851 / ztx 951 have good numbers and are actually available. I have used the excellent Hitachi / Renesas 2sa1084 / 2sc2546, but they are unobtainium now. I have a little stash that they will have to take out of my cold dead hands. However, the Zetex parts have lower rbb, and so probably least noise, as well as actually being for sale.

Decent MC stages have low impedances and high currents.
 
The original devices BC461 / BFX85 are medium signal parts (1A) parts in TO39 cans, so obviously an older part. So I would guess a reasonably low value for rbb, but would worry about the 1/f noise level. They have very low Hfe values (40 and 70) respectively, so relatively high base currents will flow the cartridge.
 
Still can figure how it works with basses connected together... it's a bit like half of two current mirrors but that's all I see really.
 
Look a lot of early battery headamps, the transistors operate in a weird way, with Voltage between Base and emitter, but hardly any between base and collector. The devices are more like controlled resistors, than controlled current sources. There was a thread a few weeks ago about the Ortofon head amp in the classic forum which might help. Still a nasty circuit.
 
Thanks again for your ace comments PD. I know you put accuracy and low noise first, but I like those feedback capacitors as they’re a great opportunity for voicing. I’ll have a go at simulating it, as well as looking for that Ortofon device you mention.

It’s like nothing I’ve ever seen before and I’ve no idea how it works. Almost as if it were designed by aliens; I’m half expecting Mr Newton to come stepping out from his newly-built spaceship.
Station_to_Station_cover.jpg
 
PigletsDad said:
Here is a link to the Ortofon discussion. Ortofon MCA-10 info | pink fish media
Click to expand...
I don’t like the Ortofon circuit (MCA-10), as it’s capacitively coupled and I wanted to avoid that.
a.palfreyman said:
"Moving Coil Cartridge Head Amps" https://leachlegacy.ece.gatech.edu/headamp/
Click to expand...
An interesting article thanks. The floating battery prevents current flow through the (direct coupled) cartridge. That didn’t occur to me. I like the fact that there are no capacitors in the signal path of the Lentek, only feedback capacitors, which I also like.
 
Avon said:
I like the fact that there are no capacitors in the signal path of the Lentek, only feedback capacitors, which I also like.
Click to expand...

Feedback capacitors are very much in the signal path! For example, if the capacitance depends on applied voltage (like it does in electrolytics) you will get phase modulation of low frequencies.
 
PigletsDad said:
Feedback capacitors are very much in the signal path! For example, if the capacitance depends on applied voltage (like it does in electrolytics) you will get phase modulation of low frequencies.
Click to expand...
Yes, but I think that gives the artificial bass slam heard in Naim amps. I’m not one for accuracy!
 
There are two circuits here (RIAA Phono Preamps (sound-au.com)) at Eliott Sound Products along with a note about the Marshal Leach common base circuit:
'There are quite a few moving coil 'head amp' circuits on the Net, but some are wildly over-complicated and others are simple, but suffer from a variety of problems. One that looks attractive at first is a design by J. Marshall Leach, which uses a pair of transistors operated in common base mode. The input goes to the emitters of the two input devices, and the supply is a 9V battery. Unfortunately, the gain of the circuit depends greatly on the battery voltage, source impedance and load impedance so it's not a design that can be recommended. This is a pity, because the circuit is very simple and probably works quite well, but if the gain changes as the battery discharges this makes the circuit useless. Obtaining accurate channel matching is likely to be a great challenge, and each channel needs its own battery.'
Must admit that I was under the impression that transconductance amplifiers were dependant on source current (impedance) anyway.
This is also worth a read: Project 187 (sound-au.com)
 
So if you don't mind me asking @PigletsDad what circuit would you build as a head amp if some of the above are compromised?
I had some ztx851 and have acquired some ztx951.
Initially it would just be an exercise, for fun more than anything. :)
 
Let me find something in my files.

I have two options in mind; one has differential pairs at the input (which costs a bit of ultimate noise performance) but means no big caps are needed, the other uses electrolytic feedback caps but has a single ended input stage. In theory, this is 3dB quieter, but in practice this gets diluted by other noise contributions so it isn't such a big advantage. A third option, which I have simulated but never built, uses the LSK389 dual FET at the input. It won't be as quiet as the bipolar designs, but has MUCH less DC current flowing through the cartridge. The differential pair one is basically the input block of the phono pre-amp I published here years ago. The one with the feedback caps has a few things in common with the JLH circuit, but is fundamentally different.

Easy enough to vary the gain to fit different cartridges. At one end you have things like the Mk 1 Ortofon MC20 at about 70uV, while say an AT OC9 is about 400uV (and I think earlier variants of that were even hotter), a range of at least 6:1. So for the very low output ones you probably want say x40 (32dB) grading down to something like x8 (9dB) for the higher outputs. Using too much gain is just asking for overload trouble and a cramped volume control.
 
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