🙂 Maybe you shouldn't call it "overkill" but "high end", then BD139/140 etc. would also fit 'in terms of feeling'. 😀
I think that if you install OREAD in an integrated amplifier, for example, you might not necessarily need a buffer output and can adjust the RIAA feedback network a little to fit for the subsequent stage, if this stage is not too low-impedance.
If OREAD is in its own housing as a pure phono preamplifier, a higher output load capacity through a buffer is an advantage, since you have to expect that it may happen that you or someone else connects lower-impedance amplifier inputs at some point, whether intentionally or not.
I think that if you install OREAD in an integrated amplifier, for example, you might not necessarily need a buffer output and can adjust the RIAA feedback network a little to fit for the subsequent stage, if this stage is not too low-impedance.
If OREAD is in its own housing as a pure phono preamplifier, a higher output load capacity through a buffer is an advantage, since you have to expect that it may happen that you or someone else connects lower-impedance amplifier inputs at some point, whether intentionally or not.
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Here is another buffer circuit, with metal can transistors (BC107/177, TO18; BC140/160, TO39).
A setup with this would also match visually the OREAD NPN version (as shown in post #371, with BC107/177); if one value something like that. 🙂
It would be capable of driving a 100 Ohm load with minimum 500mW at 24Vpp.
Input Impedance: >200kOhm (dependig of R0 and frequency)
.asc file below, including models
A setup with this would also match visually the OREAD NPN version (as shown in post #371, with BC107/177); if one value something like that. 🙂
It would be capable of driving a 100 Ohm load with minimum 500mW at 24Vpp.
Input Impedance: >200kOhm (dependig of R0 and frequency)
.asc file below, including models
If you actually use a 100 ohm load, the 220µF cap is necessary to keep the drop in the frequency response in the lower range below 0.5dB.
When it comes to the load capacity: I had in mind that one could eventually also use it to power headphones.
When it comes to the load capacity: I had in mind that one could eventually also use it to power headphones.
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Shouldn’t we then rather think about a sensible, discrete headphone amplifier?
Posting #206
can act as a buffer too.
Posting #206
can act as a buffer too.
Now I'll again use You to learn from 🙂 This is not an opinion, just put as a statement, hoping for Your comment.
Many phono preamps use a jFET input stage, e.g the Kaneda. I understand it is for the high input resistance and gain. In OREAD You have used normal pnp devices in parallel with, in my ears, excellent results. To introduce FET's in the output stage, where the signal already is decent, seems unnecessary. Isn't the risk of radio interference higher with fet devices? To that, they can be harder to come by, more sensitive to handling and even more expensive?
Or is the benefit of the high input resistance worth it?
Radio interference is typically higher with bipolar devices as they use an actively forward biased rectifying junction, and rectification can demodulate radio signals into the audio band. FETs use reverse biased junction (or MOS capacitor structure) which is less rectifying. All will rectify if pushed hard of course.
Let's put it this way:
Unfortunately, we can only outline these topics here in a rather simplified way - and that leads to inaccuracies, especially when gray areas and border areas suddenly overlap.
Mark T. has just touched on one of these areas. Anyone who is somewhat professionally minded or knowledgeable does this in the field of DIY. Me too.
From this point of view, it is difficult for me to give you a comprehensive answer and not a generalized short sentence.
But,
if you run ahead, sticking solely to the idea of the MM generator internal resistance and assuming no capacitive load on the MM, then the low-noise JFet, with its huge R_gate to source (dynamic and static) is ideal. If possible, directly behind the tonearm cabling or even as part of the headshell.
Why? Because in a source consideration there is now a voltage source with TP1 behavior. The input of the amplifier has virtually no load on the driving source. And the upper cut-off frequency (of the source) is now far outside the audio range, very far. We ignore mechanical phenomena in this consideration.
Of course, the same also applies to a subsequent buffer - huge input resistance. It is all solely a question of the load, including the load capacity.
The fact that many of the parameters are not constant or even linear must be ignored here (in the first instance/because of the necessary simplifications).
The question is:
how much is how much, how much bigger can or must it be? The RF story is a completely different matter!
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OREAD does not necessarily need this,
catd has submitted two adapted dimensioning proposals for dealing with Rl<100kOhm (or something like that).
A buffer using Fet is also just a simple matter. RF interference problems are not to be expected at this point in the signal processing.
Thank You both! Sorry for maybe wasting Your time. I'm just curious on why different design approaches are chosen.
This is quite intriguing. One problem would be supplying the amplifier. If one used some surface mounted dual opamp not on a PCB with a single suppply at the head shell? The tone arm wires could be used as ground, supply, left and right channels. For my OM pickup there's a counter weight that probably is in the same region as the necessary components mass.
This has most certainly been done, has anybody experienced something similar? I know there is some optical pickup on the market, but I don't know how much electronics there is at the head shell.
Finally, today I built the oread pair into my turntable chassis, because of the above discussions on interconnect capacitance. The connection from the tonearm to the OREAD input is a mere decimeter. The power supply has to stay outside. I have some issue with grounding. Can be because only the tonearm ground is connected to oread GC, not the turntable chassis itself. It's most prominent when I touch the tonearm. Another suspicion is that the motor controller might be picked up by oread because there is some hovering, vairating low frequency artifact that wasn't there before. Maybe some shielding is needed.
But listening wise it's the best this turntable has sounded ever for me. I used the Dire Straits Brothers in arms album because that was the reference when the CD was introduced and I started listening in a more focused manner. I know it quite well. Open is the word that comes to mind. Like the speakers are further apart and the sound is more detailed. I've ordered the test record @hbtaudio suggested from jpc and two more to listen to when I've figured out where the hum comes from. Workday tomorrow, Cheers
But listening wise it's the best this turntable has sounded ever for me. I used the Dire Straits Brothers in arms album because that was the reference when the CD was introduced and I started listening in a more focused manner. I know it quite well. Open is the word that comes to mind. Like the speakers are further apart and the sound is more detailed. I've ordered the test record @hbtaudio suggested from jpc and two more to listen to when I've figured out where the hum comes from. Workday tomorrow, Cheers
- On the test lp you will find some brute deflections - so be careful with some of the tests and read the notes on the cover
- All-round shielding will be a challenge, especially if a 2-phase synchronous motor is in the vicinity
Picture Book ... by Simply Red
&
Brothers in arms ... by the Dire Straits
😎
greetings,
HBt.
@Nick Sukhov
The expert for the installation of an MM-EQ in the chassis of the TT should be Nick S., he propagates this procedure.
I have only done this once before, with an original Supra1982, over 30 years ago. It worked great, but the belt drive used a real DC high-speed motor.
Maybe Nick S. can help better than I can - in this case.
This case actually calls for a tiny SMD-equipped circuit board ..! where is MartinX ?
😉
The expert for the installation of an MM-EQ in the chassis of the TT should be Nick S., he propagates this procedure.
I have only done this once before, with an original Supra1982, over 30 years ago. It worked great, but the belt drive used a real DC high-speed motor.
Maybe Nick S. can help better than I can - in this case.
This case actually calls for a tiny SMD-equipped circuit board ..! where is MartinX ?
😉
That warning is appreciated!
If memory doesn't fail me - The Thorens motor is a sychronos three phase construction driven by a Wien oscillator, all analog. There's a matrix of 3x3 trim potentiometers to adjust it 🙂 Speed is controlled from the from panel by observing a mains driven stroboscopic light. I believe to have read it's a known symptom that the speed starts to fluctuate when the capacitors of the oscillator age. Using ancient equipment is an endless struggle, but somehow I like that
I need to pick Simply Red from the shelf next time 🙂
cool
it's a great pleasure
that brings back a lot of memories, right back to my childhood, when I was still playing the fantastic children's adventure LPs on various German TT or my mother's single collection ...
A tape recorder and a record player, a transistor radio, but no television, that's how it was back then.
#
But now back to the mature OREAD 🤺
HBt.
it's a great pleasure
that brings back a lot of memories, right back to my childhood, when I was still playing the fantastic children's adventure LPs on various German TT or my mother's single collection ...
A tape recorder and a record player, a transistor radio, but no television, that's how it was back then.
#
But now back to the mature OREAD 🤺
HBt.
My grounding problem obviously was the turntable chassis. By connecting its chassis to my amplifier power supply chassis all hum vanished. I'm hearing artifacts in Terence Trent D'Arby's voice, or the snare drums, I've never noticed before.
The turntable is not grounded in itself. So just changing the power cord to one with ground connected to the chassis maybe would do the trick. Then I can proceed with boxing the OREAD power supply, and see if further shielding of the OREAD boards inside the turntable is needed.
I don't want to go to sleep or work tomorrow. I'd rather pour a beer, turn down the lights and continue listening
The turntable is not grounded in itself. So just changing the power cord to one with ground connected to the chassis maybe would do the trick. Then I can proceed with boxing the OREAD power supply, and see if further shielding of the OREAD boards inside the turntable is needed.
I don't want to go to sleep or work tomorrow. I'd rather pour a beer, turn down the lights and continue listening
If you put a grounded power cord on the turntable and keep the turntable ground connected to the power amp, you will have a ground loop. Make sure the power amp has a grounded power cord and then you will only have one connection to ground.