Christer said:
No, I don't think that's the main purpose, but more -as Carlosfm said before- about filtering to the next stage AND to prevent leakage of dirties from the next stage via ground if the resistor is too low. In my opinion, this can be especially true for fast and wide bandwith opamps and probably much less in the example from the scheme.
I did not put all details on the schematics, but according to the same "receip" the supply decoupling caps should be 47uF (not bigger!), bypased as usual with 100nF or so, and just above the supply voltage.
Actually, lots of thing were already described here by Carlos, and this can be some kind of "proof"
via
Christer said:
You must predict a 10k input impedance on the next stage as normal, and you must also predict that it can be much higher.
So, 100k//10k = 9K
Now suppose that the next stage has 100k input impedance.
100k//100k = 50K
It's a huge difference, isn't it?
If you didn't have the 100k resistor after the coupling cap, the difference would bw even bigger, because the cap could 'see' 10k or 100k.
So, that 100k resistor after the coupling is also part of the high-pass filter, and it is fine there, it has a purpose.
Christer said:
Did you understand my post?
What you say it true, but without the 100k resistor after the coupling cap, that difference could be 10 (10k to 100k), which is even worse.
You must not size the resistor and cap for the low-pass filter you need, you have to count with the next stage!
That's why some tube preamps have no bass when used with some SS amps, some use a 0.47uF cap on the output, followed by a 1M resistor to ground. Imagine this pre connected to a power amp with 10k input impedance.
carlosfm said:
Did you understand my post?
What you say it true, but without the 100k resistor after the coupling cap, that difference could be 10 (10k to 100k), which is even worse.
You must not size the resistor and cap for the low-pass filter you need, you have to count with the next stage!
That's why some tube preamps have no bass when used with some SS amps, some use a 0.47uF cap on the output, followed by a 1M resistor to ground. Imagine this pre connected to a power amp with 10k input impedance.
Did you understand my post? Do you really want a higher corner frequency for the HP filter? usually we want it to be low and we want to avoid using a bigger capacitor than necessary. If the following amp has 10 k input impedance, you can't get more bass by using that resistor, you will only get less bass.
Carlos, I've just noticed the suggested voltage of ±18.
Is that important for the sound of this 627 chip compared to ±15V? That's not so easy to achieve. Maybe a new Tx and reg's, unless discrete and variable, and certainly new caps needed.
And the 100k input resistor after the pot, would that be a linear pot with the 100K as law faking?
If I wanted a gain of 6, or unity, and changed the values, would that have an adverse effect of the sound of this chip, or is it just important that the value are of the order you've suggested?
I presume as an I to V, they are always at unity? I don' t know.
Thanks.
Is that important for the sound of this 627 chip compared to ±15V? That's not so easy to achieve. Maybe a new Tx and reg's, unless discrete and variable, and certainly new caps needed.
And the 100k input resistor after the pot, would that be a linear pot with the 100K as law faking?
If I wanted a gain of 6, or unity, and changed the values, would that have an adverse effect of the sound of this chip, or is it just important that the value are of the order you've suggested?
I presume as an I to V, they are always at unity? I don' t know.
Thanks.
Christer said:
No, you still don't understand!
Worst case is 9k, so a 4.7uF cap is fine.
Without a 100k resitor to ground, following our example, that cap could see on the next stage a 10k to 100k.
A 4.7uF cap with 100k to ground is not very effective in removing DC.
With 100k resistor to ground next to it, the cap will see (total) 9k to 50k.
Isn't it better? At least the range is smaller, the error is smaller.
Christer, I don't know how to explain myself better.
OK, Carlos, maybe I am starting to understand what you mean. You are actually worried about getting a too low corner frequency of the filter? Not that it will be too high? I thought you said earlier you were worried about getting too little bass, but maybe I misunderstood you.
In that case I agree with you, and that is exactly what I meant earlier that the resistor will give a higher but more well defined corner frequency. I did however suggest that was probably not the reason for the resistor, but if that is actually what you want it for, then of course it is intended to have that purpose in your circuit (when there is a coupling capacitor). However, I am not so sure that is normally the reason such a resistor is used.
Do we agree that without the resistor, a low input impedance will make the filter cut at a higher frequency and a high input impedance will make it cut at a lower frequency? Furthermore, whatever the input impedance is, the resistor will move the frequency upwards to a higher value?
In that case I agree with you, and that is exactly what I meant earlier that the resistor will give a higher but more well defined corner frequency. I did however suggest that was probably not the reason for the resistor, but if that is actually what you want it for, then of course it is intended to have that purpose in your circuit (when there is a coupling capacitor). However, I am not so sure that is normally the reason such a resistor is used.
Do we agree that without the resistor, a low input impedance will make the filter cut at a higher frequency and a high input impedance will make it cut at a lower frequency? Furthermore, whatever the input impedance is, the resistor will move the frequency upwards to a higher value?
IanAS said:
What, is it hard to make a +/-18V PSU?
The higher voltage the better, between 15 and 18V is ok.
But the biasing resistor will be chosen depending on the PSU voltage you have.
Bias at a little below 10ma.
IanAS said:
No, that 100k resistor is a much bigger value than the pot, it is only there for protection, in case the pot fails.
IanAS said:
It's important to use small value resistors, don't go much higher than 4.7k for feedback.
IanAS said:
As I/V they just convert the voltage to current, no gain.
Christer said:
Of course it will, but you must count with that and dimention the cap.
Look at the 4.7uF cap example.
9k is fine, isn't it?
50k is not great, but it's still ok.
A 10k to 100k range is much worse than this.
Of course, the resistor after the cap must be at a value that doesn't change the impedance much when paralleled with the next stage, when the next stage is low impedance.
Christer, there are always compromises, because there are no standards.
You just deal with those compromises, trying to make them have less impact, if you want the preamp to be more or less 'universal'.
Carlos,
Yes I am getting what you mean now. As I said, I also realized that effect in my earlier post, but maybe I drew the wrong conclusion when I suggested that was not usually the reason for such a resistor. Since we must anyway dimension the cap for the lowest input impedance, I agree it can make sense to add this resistor to reduce the dependency on the input impedance somewhat. It won't have a big effect, but it doesn't cost much so I agree it makes sense to use it in this case, that is, when we have a coupling cap.
I misunderstood you earlier. All the time I though you were trying to get a lower corner frequency by adding the resistor, which won't work, of course, and of course you realized that too.
EDIT:
This is also shows why it is better to have coupling caps at the inputs of units than at the outputs, if there is a choice. On the input side we know the input impedance, and the output impedance can almost always be assumed very low for SS equipment.
Yes I am getting what you mean now. As I said, I also realized that effect in my earlier post, but maybe I drew the wrong conclusion when I suggested that was not usually the reason for such a resistor. Since we must anyway dimension the cap for the lowest input impedance, I agree it can make sense to add this resistor to reduce the dependency on the input impedance somewhat. It won't have a big effect, but it doesn't cost much so I agree it makes sense to use it in this case, that is, when we have a coupling cap.
I misunderstood you earlier. All the time I though you were trying to get a lower corner frequency by adding the resistor, which won't work, of course, and of course you realized that too.
EDIT:
This is also shows why it is better to have coupling caps at the inputs of units than at the outputs, if there is a choice. On the input side we know the input impedance, and the output impedance can almost always be assumed very low for SS equipment.
Thanks Carlos for your information
The PSU I have cost a few £. The Tx cost me £100, a custom made one off to a special spec of my own and the Tx company. Then there the OsCon and Black gate 16 Volts caps. The bank of Nichicon KZ and under the PCB the BG 4700mic @ 35V would be OK as though. I think th TX is making enough for 18V though the discrete regs, just they'd need new resistor and zenner values. Though i'm not sure all chips are happy at that and i don't have regs for each chip with this DAC. No preamp at the moment, just CD to source select to Amps. There is a pic in my profile, the top right is the output stage and DAC supply, lower right is digital supply with a 1000VA tx borrowed from an Amp as it had 12-0-12 extra 2ndaries.
Re the coupling caps dialogue:
My findings are that coupling caps and HiFi are mutually exclusive terms.
Even non inductive foil poly prop caps. Even Black Gate NX. Infinicaps.
Actually, i've not tried an OsCon SEP as coupling yet, but they are polarised, so possibly at a disadvantage straight away.
One thing the output resistor to earth can possibly do is mitigate the effects of capacitance and inductance of the line (inter connect cable) on the output stage and feedback of an OpAmp.
The PSU I have cost a few £. The Tx cost me £100, a custom made one off to a special spec of my own and the Tx company. Then there the OsCon and Black gate 16 Volts caps. The bank of Nichicon KZ and under the PCB the BG 4700mic @ 35V would be OK as though. I think th TX is making enough for 18V though the discrete regs, just they'd need new resistor and zenner values. Though i'm not sure all chips are happy at that and i don't have regs for each chip with this DAC. No preamp at the moment, just CD to source select to Amps. There is a pic in my profile, the top right is the output stage and DAC supply, lower right is digital supply with a 1000VA tx borrowed from an Amp as it had 12-0-12 extra 2ndaries.
Re the coupling caps dialogue:
My findings are that coupling caps and HiFi are mutually exclusive terms.
Even non inductive foil poly prop caps. Even Black Gate NX. Infinicaps.
Actually, i've not tried an OsCon SEP as coupling yet, but they are polarised, so possibly at a disadvantage straight away.
One thing the output resistor to earth can possibly do is mitigate the effects of capacitance and inductance of the line (inter connect cable) on the output stage and feedback of an OpAmp.
This has been a very educational thread for me. Thanks for the good discussion, everyone.
Here's a relevant link that I found to be instructive:
http://tangentsoft.net/audio/opamps.html
Seems like a good comparison between a lot of popular opamps.
Here's the index page:
http://www.tangentsoft.net/audio/
Best,
KT
Here's a relevant link that I found to be instructive:
http://tangentsoft.net/audio/opamps.html
Seems like a good comparison between a lot of popular opamps.
Here's the index page:
http://www.tangentsoft.net/audio/
Best,
KT
carlosfm said:...I also like very much the OPA847, as I reported several pages back.
But it's unfair... for a preamp, the AD815 eats everything for lunch.
PS: I've seen (and heard) several so-called 'high-end' preamps and integrated amps with the OPA604 and I just wanna
Carlos,
Did you mean AD847 and not OPA847? I saw that you recommended AD843 and AD847 quite enthusiastically, but I found no mention of OPA847.
Thanks,
KT
Good price on AD815
Hey guys,
This seems like a good deal on the AD815.
http://tinyurl.com/agy55
It's listed as AD815AYS. I hope it's usable in Carlos' preamp circuit.
Best,
KT
Hey guys,
This seems like a good deal on the AD815.
http://tinyurl.com/agy55
It's listed as AD815AYS. I hope it's usable in Carlos' preamp circuit.
Best,
KT
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.