| Best MC Load Resistor Type, current gain stage... - Click HERE for Original Thread |
| tubenut |
Hi all
I am inviting opinions.
My phono pre (Electrocompaniet) uses current amplification.
I messed a little with the input load resistor value (10R) onlly to go back to the original value of 10R. I used a fresh commercial grade metal film resistor. The sound just seemed wrong.
Replaced with the original "used" resistors and a all was well.
So what is your favourite load resistor type:
Vishay Bulk MF?
Tantalum?
Precision Carbon?
Any number of Metal Films from Holco, PRP etc?
I guess experience with passive I/V CD stage resistor types would count here as well.
Ta |
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| SY |
| Bulk metal foil in that position. |
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| anatech |
Hi SY,
Is there that much of a difference? Naked foil or bare (I'm guessing sealed / coated)?
There can be differences in metal film types that are measurable.
-Chris |
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| SY |
| The ones I have are sealed in little square epoxy packages. I'm not sure of the brand- got 'em as a gift. |
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| anatech |
Sy,
You're one lucky fella. Good going! ;)
One fella was recommending the naked bulk metal foil over the encapsulated ones because "he could hear a difference". Physiology is a wonderful thing. They cost more he says, that must be it!
That's like having a naked capacitor! Okay, low voltage.
-Chris |
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| AndrewT |
Hi,
if a wire (non inductive wound) resistor were made, what material should sound best?
Copper, constantan, nichrome? Others? |
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| sreten |
Hi,
For a virtual earth current amplifier the best load resistor is none at all !
:)/sreten. |
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| anatech |
Hi sreten,
Can't argue that, except you still need a sensing element. That would be doped silicon in most cases (ultimately).
-Chris |
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| sreten |
Hi,
To be accurate the cartridge internal resistance becomes the "load"
resistor and the virtual earth current stages voltage gain becomes
directly dependant on the cartridges impedance, self adjusting.
Adding a 10R series "load" resitor simply makes the gain changes
less severe as the cartridge source resistance changes, whether
this is a good idea or not depends on the internal gain setting.
:)/sreten. |
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| tubenut |
| I think the 10R sit from signal in to ground, that is parallel not series? |
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| sreten |
| quote: | Originally posted by tubenut
I think the 10R sit from signal in to ground, that is parallel not series? |
Hi,
Generally AFAIK for virtual earth inputs, input resistor is in series.
Its the standard Shunt feedback layout :
I'll let you work out what happens if the signal is injected at the
virtual earth point, i.e. R2 is connected from signal in to ground.
(If R2 is 10R , R1 say 100R, it would not change very low cartridge
R increasing the gain but would limit the reduction in gain of high
R cartridges, which is not exactly what you want I suppose.)
:)/sreten. |
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| tubenut |
Increasing the 10R reduced gain. The reason for changing it was that I thought the Denon 103 I used may be driving to low a load/impdedance and the manual for the device says MC load is 10R........ I would not mind extra gain and if indeed this resistor is in series, going to say 8R2 or 6R8 may be useful.
It is a little harder for me to trace the cct as it is not op amp based but discrete. It could be a "set of discretely built" op amps. It has a dozen or so trannies per channel.
Will ask a friend if he has a diagram....
G |
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| AndrewT |
Hi,
it would be worth spending the time reverse engineering what you have.
If you have an inverting stage then increasing 10r will reduce the gain. (and it applies to non-inverting as well)
The 10r is also the input impedance, since the inverting input is a "virtual earth".
Am I correct in thinking that inverting input is marginally quieter then non-inverting input? Just how marginal? Does this rule apply to all opamps (BJT, FET, discrete)? |
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| EC8010 |
| quote: | Originally posted by AndrewT
Am I correct in thinking that inverting input is marginally quieter then non-inverting input? |
Not quite. The input stage inside an op-amp is invariably a differential pair, so each input is identical. But what you do outside can change the noise. Adding a series resistor (as in an inverting input) increases noise because it's a noise source in series with the signal source so the two voltages add. Conversely, a non-inverting op-amp has the load resistance in parallel with the signal source, and because the signal source is typically one tenth the load resistance, the signal source shunts away most of the noise produced by the load resistance. Thus, the non-inverting configuration is quieter than the inverting configuration. The way around this problem is (as sreten suggested), not to have a load resistor at all in the inverting configuration. |
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| AndrewT |
Hi,| quote: | | Thus, the non-inverting configuration is quieter than the inverting configuration |
I don't think this is the correct conclusion. |
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| forr |
SRETEN
---For a virtual earth current amplifier the best load resistor is none at all !---
I've seen such a transresistance circuit for a microphone. It has then been severely criticised for giving an inadequate electrical damping. I am not aware of microphones or MC cartridges conceived to be loaded by a null impedance, which means as current sources. |
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| sreten |
| quote: | Originally posted by tubenut
Increasing the 10R reduced gain. The reason for changing it was that I thought the Denon 103 I used may be driving to low a load/impdedance and the manual for the device says MC load is 10R........ I would not mind extra gain and if indeed this resistor is in series, going to say 8R2 or 6R8 may be useful.
G |
Hi,
For current sensing all "normal" loading principles go out of the window.
You can completely omit the resistor, gain will be set by cartridge resistance.
:)/sreten. |
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| tubenut |
Will try no resistor and see....
PS, there are no op amps inside my EC, it is discrete.... |
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| EC8010 |
| quote: | Originally posted by AndrewT
Hi, I don't think this is the correct conclusion. |
Why not? The inverting configuration adds the voltage noise source of the load resistor in series with the source, whereas the non-inverting puts it in parallel. |
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| sreten |
| quote: | Originally posted by forr
SRETEN
---For a virtual earth current amplifier the best load resistor is none at all !---
I've seen such a transresistance circuit for a microphone. It has then been severely criticised for giving an inadequate electrical damping. I am not aware of microphones or MC cartridges conceived to be loaded by a null impedance, which means as current sources. |
Hi,
virtual earth loading gives maximum damping, so inadequate electrical
damping doesn't make any sense. For 600ohm microphone circuits
and long cables virtual earth loading represents mis-termination of
the 600 ohm line, and I presume it would not work well.
MCs are not concieved to be current sources, that would fly against
the type of most active inputs and obviously does not suit the basis
of matching transformers. However that does not mean it won't work.
A virtual earth MC input has one "convenience", it automatically
adjusts its gain to suit the MC, assuming the MC follows the
basic principle of the higher its output, the higher the source R.
The main problem with including it in a pre-amp is explaining
to the pre-amp owner how it effectively works and thus what
cartridges can be used with the MC input.
:)/sreten. |
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| Gretsch6136 |
Hi Tubenut,
You may have some information (or opinion) That I have been searching for.
I have the Electrocompaniet ECP 1 phono stage. My understanding is that it is set up with a 10 ohm input impedance.
My cartridge is the Denon DL304 which has a 40 ohm output impedance and a 100 ohm load impedance (whatever that means).
I have been told by others on this forum that this mismatch would cause me to be experiencing a major loss in gain. Also that the cartridge would not sound right, that it would be thin and flat sounding.
What are your thoughts and would you recommend changing the input impedance of the ECP1? If so, have you an opinion on what impedance it should be changed to??
Thanks,
Mark |
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| tubenut |
Hi Mark
| quote: | For current sensing all "normal" loading principles go out of the window.
You can completely omit the resistor, gain will be set by cartridge resistance.
/sreten. |
My Denon 103 works fine and the ecp1 has sufficient gain for, having played with a Lehman Black Cube over the weekend the levels from either stage (the Cube can be set for all kinds of carts and has a place for a "custom" resistor value on the PCB) are similar.
Guess it is just that Theta DAC in Balanced mode that is so much "louder" then everything else....
I do not believe EC would be dumb enough to make a phono pre that only works witha handfull of carts, as sreten said, the cartridge sets the input impedance and gain and in a way is self adjusting... The 10 Ohm is probably in series not parallel and calling it a load resistor is I suppose a misnomer.
Do not worry about your cart as obviously this stage does not work in the same way as all the others...
Perhaps EC should have done a bit more explaining about the merits and workings of current amplification in the manual...
Do yourself a favour though, get a bigger transformer hooked up to the ECP1, say 80 VA at 40 Volts AC in to the ECP1, you will not believe the improvement, I would say in excess of 30%!!!
The factory supplied mickey mouse wall wart 10 VA supply really lets the ECP1 down.... |
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| AndrewT |
Hi Tubenut,
can you confirm the transformer voltage that is required to replace the wallwart type? |
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| tubenut |
I think the original wall wart was 250 mA @ 42 Volt AC
I run my ECP of a toroidal Txf, 220 V Primary, 40 V Secondary @ 80 VA. The higher current rating probably gives the extra few volts anyway as less then 1/8 the available current is drawn from it....
Before I used a conventional block txf 220 V Pri, 20-0-20 Sec wired to 40 V also at 80 VA.
Cannot say which is better, there was a 6 week break between the block and toroid but both kill the wallwart...
The ECP1 takes AC in so there is no real complicated stuff here. |
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| Gretsch6136 |
Hi Tubenut,
You may be interested to know that I printed off this thread and took it to my tech.
He agreed with Sreten that the input resistor could be ommitted from the ECP1 altogether due to its current amplification design.
Well I just got the unit back from him, and boy has this made a difference. I listened to Pink Floyd's Delicate Sound of Thunder live album. The difference is like night and day.
The sound now has a much bigger soundstage, more three dimensionality, lots of air and separation. The dynamic response is lightning quick and the bass extension has been improved. There is also more gain so I don't have to turn the amp up as much either.
The best $30 I ever spent!!
Cheers,
Mark |
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| tubenut |
Hi Mark, thanks for sharing that and it is most certainly good worthwhile info.
Also glad the thread has helped someone else and proves I was not going insane when I heard huge differences between different 10R resistors in that place...
I will probably bypass mine over the weekend... Now what to use? Cut off resistor leg? OFC copper wire, some silver I have kicking about?
Choices....
Mark, if you have not already done so, run the ecp of a bigger transformer as well!!! |
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| Gretsch6136 |
Hi Tubenut,
My tech made a couple of jumpers out of Kimber cable.
As for the power supply upgrade, I am keen to head down that path too. Is there an off-the-shelf replacement you can recommend or do I need to get my tech to make something up?
Thanks,
Mark |
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| AndrewT |
Hi,| quote: | | bypass mine over the weekend... Now what to use? |
a DIP switch :eek: |
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| tubenut |
Hi all
Well, last night the 10R resistors came out and silver wire went in (I have used the same stuff from RCA to PCB already.)
I would agree it is an upgrade with better dynamics bass depth and staging, I just need to get used to the slightly more forceful sound, perhaps a hint of a little hardness.
WRT to the PSU:
I do not know if there is something off the shelf available in your habitat...
You need a say 50 VA or greater 40 Volts AC supply. Perhaps there is something out there but it would probably be a spare part for something else.
Your techie could make this up but keep in mind you will be paying him for drilling holes in a case rather then his expertise as making the box is the bigger job. Wiring it all up takes less then 30 mins.
Perhaps you should get:
40 Volt transformer @50 VA or more
A box the TXF fits in
Some Mains Power Cable
Gland for the incoming power cable
Fuse Holder
Suitably rated fuse
A wire for outgoing power, a thin Taiwanese OFC speaker cable is good, say 18 gauge
A Gland for outgoing power
One of those low voltage power plugs to go into the back of the ECP1, take the ECP1 along as there are many sizes that are visually close....
Then drill the relavent holes and mount al the bits and then take it to your techie to wire up......
Best |
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| sreten |
Hi,
just point out once you have removed the input resistor then the
next thing to point your finger at is the quality of the feedback
resistor. Its value can be reduced to reduce gain. Overload of
the MC gain stage is unlikely, more likely is clipping or slew rate
limiting in the phono stage, especially if it has passive RIAA eq.
:)/sreten. |
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