Hi Andrew,
You have a funny way of putting things.
Thank you for your detailed instructions on how to take measurements !
What would I do without them ! Well I managed to take those measurements.
These are measured at 100 hz because I'm not sure what the input capacitance of the meter is. The impedance indicated on the front panel is 10 Meg ohms.
I used an 80K R to get approx 79K. I used a 1.12Meg R to get approx 1 Meg.
open circuit voltage ( 10 Meg ) 1.00 V
79 K load 0.54 V
1 Meg load 0.94 V
Cheers.
You have a funny way of putting things.
Thank you for your detailed instructions on how to take measurements !
What would I do without them ! Well I managed to take those measurements.
These are measured at 100 hz because I'm not sure what the input capacitance of the meter is. The impedance indicated on the front panel is 10 Meg ohms.
I used an 80K R to get approx 79K. I used a 1.12Meg R to get approx 1 Meg.
open circuit voltage ( 10 Meg ) 1.00 V
79 K load 0.54 V
1 Meg load 0.94 V
Cheers.
Hi Minion,
In your case it probably doesn't matter at all but did you try feeding the preamp output directly to the power amp ?
Why don't you do that and see how it 'sounds'. You might not need the buffer at all ! Try it.
Well if the buffer is to take care of a 'long' interconnect to the power amp I guess it's required. Preamp being near the guitar of course (?). If the buffer is 'at' the power amp it doesn't help unless your preamp cannot work with impedances like 10 K ohms.True that tube preamps have greater distortion with lower impedance loads but practically are they audible in this case ? But with the HF limits of a guitar a high Zout shouldn't matter if the voltage levels are met !
One thing that I had forgotten to look at was that as the preamp output impedance started rising the value of the coupling capacitor became less critical. I could easily get away with a 1uF cap on 8 K loads . With a low output impedance preamp I would get significant loss of the low end with a 1uF and 8 K load.
Another effect was that there appeared to be some difference in the low end. Like I mentioned earlier the high Zout preamp seemed to have a more 'meaty' bass end. Punchy drums had a tad more weight not really explaind by the difference in response ( about -0.6dB ).
The slight ( audible ) HF loss disappeared when the Zout was reduced but still high ( about 16K ohms !). So apparently it wasn't the coupling cap. Now on to trying botique caps . This will have to wait. I'm away for about 10 days and possibly with no chance to get on to the NET when I'm away! That sounds silly as I'm going to Mumbai ! But not carrying my laptop etc. Severe checks etc. while travelling as they are expecting trouble from those hoodlums who like to blow up innocent people.Laptops are highly suspicious gadgets nowadays ! Gad , what levels have we sunk to !!!!!!!!!! But.....if you never see a post from me again .........you know what happened.........I must have been assisted to reach Heaven (?) faster ! I'd miss DIYaudio ! ( the last big bang was on 26/11/2008 .......they apparently want to celebrate with one or more bangs ! ....)
To those of you who added unity buffers to their tube preamps , take them off or by pass them and see how the system sounds. For some it might be a revelation and others with 'configuration needs' it might not be so.But you need to try to find out. This is a DIY forum...huh !
Just listening to the best of Dave Grusin through my high Zout preamp and a few glasses of Rum ! Heavenly ! It's almost 11pm here and I must log off and listen to some more music before I crash out and frantically pack ( tomorrow ) to leave for Mumbai ......
Cheers.
edit: Hey I saw a post from Minion before replying. But after I wrote out this long reply the original short post by Minion has disappeared . Why ? I ain't so drunk ...not yet !
Edit2: Yeah I must be drunk ! Minion posted on page 1 and we are on page 3 ! .......Ha.....
In your case it probably doesn't matter at all but did you try feeding the preamp output directly to the power amp ?
Why don't you do that and see how it 'sounds'. You might not need the buffer at all ! Try it.
Well if the buffer is to take care of a 'long' interconnect to the power amp I guess it's required. Preamp being near the guitar of course (?). If the buffer is 'at' the power amp it doesn't help unless your preamp cannot work with impedances like 10 K ohms.True that tube preamps have greater distortion with lower impedance loads but practically are they audible in this case ? But with the HF limits of a guitar a high Zout shouldn't matter if the voltage levels are met !
One thing that I had forgotten to look at was that as the preamp output impedance started rising the value of the coupling capacitor became less critical. I could easily get away with a 1uF cap on 8 K loads . With a low output impedance preamp I would get significant loss of the low end with a 1uF and 8 K load.
Another effect was that there appeared to be some difference in the low end. Like I mentioned earlier the high Zout preamp seemed to have a more 'meaty' bass end. Punchy drums had a tad more weight not really explaind by the difference in response ( about -0.6dB ).
The slight ( audible ) HF loss disappeared when the Zout was reduced but still high ( about 16K ohms !). So apparently it wasn't the coupling cap. Now on to trying botique caps . This will have to wait. I'm away for about 10 days and possibly with no chance to get on to the NET when I'm away! That sounds silly as I'm going to Mumbai ! But not carrying my laptop etc. Severe checks etc. while travelling as they are expecting trouble from those hoodlums who like to blow up innocent people.Laptops are highly suspicious gadgets nowadays ! Gad , what levels have we sunk to !!!!!!!!!! But.....if you never see a post from me again .........you know what happened.........I must have been assisted to reach Heaven (?) faster ! I'd miss DIYaudio ! ( the last big bang was on 26/11/2008 .......they apparently want to celebrate with one or more bangs ! ....)
To those of you who added unity buffers to their tube preamps , take them off or by pass them and see how the system sounds. For some it might be a revelation and others with 'configuration needs' it might not be so.But you need to try to find out. This is a DIY forum...huh !
Just listening to the best of Dave Grusin through my high Zout preamp and a few glasses of Rum ! Heavenly ! It's almost 11pm here and I must log off and listen to some more music before I crash out and frantically pack ( tomorrow ) to leave for Mumbai ......
Cheers.
edit: Hey I saw a post from Minion before replying. But after I wrote out this long reply the original short post by Minion has disappeared . Why ? I ain't so drunk ...not yet !
Edit2: Yeah I must be drunk ! Minion posted on page 1 and we are on page 3 ! .......Ha.....
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Bang on. In my old trade notes I noticed a comment regarding the loading of a Baxandall stage: the thd goes up roughly double with o/p load equal to anode resistance; (surely not ?) and the stage feedback is mean't to be a low Z operator.
Simple solution: Cathode-follower it for best fidelity.
richy
Actually in my guitar amp the Buffers after the tube stage are also used for boosting a cutting certain frequencies , sort of like speaker simulators to give bass and trebble boost, the pre also has a DI out so there are opamps for that , is also has an opamp at the input with adjustable gain to overdrive the tubes , I also run the pre through a 15 band eq before going into the power amp and they are also in the same chassis ...
So basicly there is no way to bypass the Buffers accept with the fx loop which is between triode stages but it has a 680k output impedance and my Power amp has a 47k input impedance so that aint gonna work.....
Dec update.
For those who read this thread I have an update.
A recap: the preamp sounded good with the last setup feeding a Rotel 'power' amp.
I have just got a TDA8290 classD module. It sounds very nice though I think the HF and voice reproduction isn't the best I have heard. Bass however is very tight and extended .
I added the preamp used above with this module. The very low bass disappeared and so did the tight bass. Voice I felt was a bit better ( more life like - 'softer' ) and treble didn't seem to change. So here the add on pre didn't help.
The TDA module has a 68K input impedance and a 1uF cap in series with the input . The preamp output cap is also 1uF. So we get an over all -3dB at 4.7Hz . Hardly any reason for the low end to disappear !
Will try to look at the FFT's when I try it next. I'll try this with a 40Hz Amp2.
For those who read this thread I have an update.
A recap: the preamp sounded good with the last setup feeding a Rotel 'power' amp.
I have just got a TDA8290 classD module. It sounds very nice though I think the HF and voice reproduction isn't the best I have heard. Bass however is very tight and extended .
I added the preamp used above with this module. The very low bass disappeared and so did the tight bass. Voice I felt was a bit better ( more life like - 'softer' ) and treble didn't seem to change. So here the add on pre didn't help.
The TDA module has a 68K input impedance and a 1uF cap in series with the input . The preamp output cap is also 1uF. So we get an over all -3dB at 4.7Hz . Hardly any reason for the low end to disappear !
Will try to look at the FFT's when I try it next. I'll try this with a 40Hz Amp2.
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