Prepreamp
Hello Frank,
answer comes late, but comes.
Have found the schematic of the MCP33 in the ARC database
at
www.arcdb.ws
Here under phono and then MCP 33 and scrolling to the end of the document.
Reinhard
Hello Frank,
answer comes late, but comes.
Have found the schematic of the MCP33 in the ARC database
at
www.arcdb.ws
Here under phono and then MCP 33 and scrolling to the end of the document.
Reinhard
I have found the MCP33 to be one of the more interesting designs coming from ARC. It is a true “grounded grid” design (unlike Rosenblitz’s so called GG, which is really better described as a diff amp), even though there is some feedback to the first stage grids. But the real point is that signal is input to the cathode (through some very large film caps) rather than to the grid, which then allows for some degree of impedance matching to the source, as well as for non-inverting operation. The upper triode section is really only a partial cascode since it tracks an attenuated version of the output signal via feedback resistors. This follows William Johnson's patent very closely. I am leery of ARC's common use of diodes or FETs to protect the grids of cathode followers, because of my concern about voltage-dependent (non-linear) capacitance smearing higher frequencies. Granted, the boot-strapping effect of the cathode keeps voltage swings across the FET to a minimum, but I'd be tempted to avoid it altogether if the power-supply start-up sequencing can be arranged to avoid stressing the cathode follower. This design represents trade-offs between the two feedback paths to optimize the compromises among gain, input impedance and noise performance. The design is pretty clever overall, although I’m not sure that it would perform all that much better than a well-conceived common cathode design. Power supply rejection is not all that good, so the power supply would have to be quite special (and ARC went to great pains to make sure that it was).
Reinhard, please keep us posted on how this project works out for you.
Reinhard, please keep us posted on how this project works out for you.
mc step up
Brian,
is this LED bias in Morgan Jones third Edition. Have not seen that, have first and second edition.
Victor from BAT uses paralled 6922, in the Aesthetix IO are two paralled 12AX7`s (4 systems) running from 550 Volt with a 100k
anode load and a 274 Ohm cathode resistor, bypassed with a big cap.
CAT uses cascoded 6922, the MFA Refrence a 6922 cascoded with a AX7 system, special Cascode with extra anode load for the 6922
Do you have a scan of the Jones circuit?
Further i am looking for the Carver in annother thread
Regards from Germany,
Reinhard
Brian,
is this LED bias in Morgan Jones third Edition. Have not seen that, have first and second edition.
Victor from BAT uses paralled 6922, in the Aesthetix IO are two paralled 12AX7`s (4 systems) running from 550 Volt with a 100k
anode load and a 274 Ohm cathode resistor, bypassed with a big cap.
CAT uses cascoded 6922, the MFA Refrence a 6922 cascoded with a AX7 system, special Cascode with extra anode load for the 6922
Do you have a scan of the Jones circuit?
Further i am looking for the Carver in annother thread
Regards from Germany,
Reinhard
The bias circuit is just a forward-biased red LED in place of the cathode bias resistor. A red LED (not the high efficiency type!) will give you about 1.7V. Once caution is that this is only suitable for current draws over 5-10mA or so, otherwise the LED impedance goes too high. For lower current circuits, there's a work-around, using a CCS from the B+ to run some extra current through the LED without upsetting the tube's operating point.
If you don't have MJ's Third Edition, you need to run right out. It's at least 50% larger than the Second. In it, he goes through LED and other reference options in gory detail.
If you don't have MJ's Third Edition, you need to run right out. It's at least 50% larger than the Second. In it, he goes through LED and other reference options in gory detail.
I agree with SY that you really ought to buy Morgan Jones' latest "Valve Amplifiers". It's a great book and and has plenty of design information on RIAA stages. I would not feel comfortable scanning anything from his book since he deserves your purchase, and there are copyright laws after all.
What is here the main disadvantage (except the fact, that more space is necessary) ?
For me this is the best solution to protect the MC cartridge coils against DC. Except the approach with a step up transformer there are no other approaches with DC protect.
Even by the use of electrolytic (polarized) caps at the input (like Mark Levinsons JC-1) I don't note losses in sonic quality.
Hi,
Every circuit has its sonic fingerprint just as all passive components do, x-formers included.
Given that this is a MC pre-preamplifier with high gain, a cap at the input will carry its sonic signature right across the entire amplification chain.
It's not that hard to avoid this input cap. Given the low output of the MC cartridge and knowing that a 6DJ8 can easily be run at Vg0 as well as at slightly positive grid voltages, you simply ground the cathode.
The reason why you may not be able to hear the impact of the input cap
may well be your use of a very low value Vgin resistor to load your cartridge. However, do keep in mind that as input resistance rises so will the audibility of the cap. A component avoided in the signal path is a good thing.
Some years ago (tempus fugit) I offered a schematic of a MC headamp which I nicknamed "MC Hammer". Not sure if all the SS parts are still around but it may give some idea of how to beat some prejudices you seem to be holding.
Me, I never heard a x-former coupled MC system that sounded truly transparent. Granted, nothing ever is but still...
Cheers,
Every circuit has its sonic fingerprint just as all passive components do, x-formers included.
Given that this is a MC pre-preamplifier with high gain, a cap at the input will carry its sonic signature right across the entire amplification chain.
It's not that hard to avoid this input cap. Given the low output of the MC cartridge and knowing that a 6DJ8 can easily be run at Vg0 as well as at slightly positive grid voltages, you simply ground the cathode.
The reason why you may not be able to hear the impact of the input cap
may well be your use of a very low value Vgin resistor to load your cartridge. However, do keep in mind that as input resistance rises so will the audibility of the cap. A component avoided in the signal path is a good thing.
Some years ago (tempus fugit) I offered a schematic of a MC headamp which I nicknamed "MC Hammer". Not sure if all the SS parts are still around but it may give some idea of how to beat some prejudices you seem to be holding.
Me, I never heard a x-former coupled MC system that sounded truly transparent. Granted, nothing ever is but still...
Cheers,
Aside from possible sonic degradation from the big input caps, I have another concern. These caps "block DC" only after the circuit has warmed up and settled to its quiescent condition. 82uF is connected between the moving coils and the paralleled cathodes of the input stage. At power up those caps have to be charged to about 1.2 volts at the cathode end. The charging current is supplied by the cathodes, but that same current must also flow through the coils of the cartridge. At power down, the reverse happens. Some of the charging current will be shunted into the input loading resistors, depending on what value of loading was selected. Still, a momentary pulse of some amount of current will flow into the delicate cartridge before "DC blocking" eventually takes place. It's obviously not enough to fuse the coil wire, but it might induce a residual magnetic charge in the armature or in other part of the cartridge's magnetic circuit. I can't say for sure that this would be a problem for every (or any) cartridge, but if I'd just dropped $5K on a fancy cartridge I might think twice about the situation.
I suppose you could add a normally-closed relay to short the input until the circuit settles, but this seems like a bandaid, and maybe not foolproof during a short power outage or brown-out.
I suppose you could add a normally-closed relay to short the input until the circuit settles, but this seems like a bandaid, and maybe not foolproof during a short power outage or brown-out.
Hi,
Every circuit has its sonic fingerprint just as all passive components do, x-formers included.
Given that this is a MC pre-preamplifier with high gain, a cap at the input will carry its sonic signature right across the entire amplification chain.
It's not that hard to avoid this input cap. Given the low output of the MC cartridge and knowing that a 6DJ8 can easily be run at Vg0 as well as at slightly positive grid voltages, you simply ground the cathode.
The reason why you may not be able to hear the impact of the input cap
may well be your use of a very low value Vgin resistor to load your cartridge. However, do keep in mind that as input resistance rises so will the audibility of the cap. A component avoided in the signal path is a good thing.
Some years ago (tempus fugit) I offered a schematic of a MC headamp which I nicknamed "MC Hammer". Not sure if all the SS parts are still around but it may give some idea of how to beat some prejudices you seem to be holding.
Me, I never heard a x-former coupled MC system that sounded truly transparent. Granted, nothing ever is but still...
Cheers,
Thank you for your advices. Is the schematic of the "MC Hammer" to find on the web? Maybe similar to this:
Moving Coil Cartridge Head Amps
In this case also this thread is of interest:
http://www.diyaudio.com/forums/tubes-valves/123010-tube-phono-stage-dilema-3.html#post3520285
Interesting advice. In this case this threads are of interest:
http://www.diyaudio.com/forums/analogue-source/229794-allowable-dc-current-phono-cartridge.html
http://www.diyaudio.com/forums/anal...-much-error-voltage-before-coil-damaging.html
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If you are the proud owner of an ARC MCP-33, you can run this little test. Look closely at your cartridge's cantilever while you flick on the power switch. If the cantilever gives you a little "Hello there" nod, you might have a problem
Of course, a "head amp" like the MCP-33 is intended to be used in lieu of an input transformer. If there were ever a situation in audio for which a transformer is ideally suited, it's probably in the MC front-end. Best possible noise performance, "DC" blocking, ground-loop stopping...
But...I laugh my evil paranoia-inducing laugh... an input transformer will not necessarily protect your MC from current pulses at turn-on or turn-off if the active circuit induces pulses of current into the secondary. Transformers and caps block "DC", yes, but as an old professor of mine once said "there is no such thing as DC in the real world".
As a more general comment, the turn-on and turn-off behavior of active circuits, tubes and solid-state, is often overlooked, even by seasoned manufacturers. And how do circuits respond to rapid power cycling and brown-outs such as a nasty thunderstorm might cause? Well, another topic for another thread...
Of course, a "head amp" like the MCP-33 is intended to be used in lieu of an input transformer. If there were ever a situation in audio for which a transformer is ideally suited, it's probably in the MC front-end. Best possible noise performance, "DC" blocking, ground-loop stopping...
But...I laugh my evil paranoia-inducing laugh... an input transformer will not necessarily protect your MC from current pulses at turn-on or turn-off if the active circuit induces pulses of current into the secondary. Transformers and caps block "DC", yes, but as an old professor of mine once said "there is no such thing as DC in the real world".
As a more general comment, the turn-on and turn-off behavior of active circuits, tubes and solid-state, is often overlooked, even by seasoned manufacturers. And how do circuits respond to rapid power cycling and brown-outs such as a nasty thunderstorm might cause? Well, another topic for another thread...
If you are the proud owner of an ARC MCP-33, you can run this little test. Look closely at your cartridge's cantilever while you flick on the power switch. If the cantilever gives you a little "Hello there" nod, you might have a problem
Of course, a "head amp" like the MCP-33 is intended to be used in lieu of an input transformer. If there were ever a situation in audio for which a transformer is ideally suited, it's probably in the MC front-end. Best possible noise performance, "DC" blocking, ground-loop stopping...
But...I laugh my evil paranoia-inducing laugh... an input transformer will not necessarily protect your MC from current pulses at turn-on or turn-off if the active circuit induces pulses of current into the secondary. Transformers and caps block "DC", yes, but as an old professor of mine once said "there is no such thing as DC in the real world".
As a more general comment, the turn-on and turn-off behavior of active circuits, tubes and solid-state, is often overlooked, even by seasoned manufacturers. And how do circuits respond to rapid power cycling and brown-outs such as a nasty thunderstorm might cause? Well, another topic for another thread...
Interesting considerations. This means in case of circuits with low impedance inputs the necessity to use the input as output (by connection to the line input of an integrated amp e. g.) for checking out the turn-on and turn-off behavior and steps to eliminate any unwanted poppings through transients.
I have sometimes heard from cracked coils in MC cartridges without having had a reason for this. Whether it was this or whether it was corrosion - that is the question.
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