Autobahn
Active Member
Wow! What a coincidence! I've had the same Preamp kit sitting around on a shelf for the last 2 years. I just assembled it a week ago, and I have the same problem. Or maybe it's just the missmatch of equipment. I had used a old Marantz CD player with it to a old Kenwood KM-992 Amp with Elac B5 speakers. Gain seems too be way to high. If 7 o'clock is minimum volume I couldn't reach 8 with out it being close to clipping and distorted. I put a pair of Rothwell Audio in-line signal attenuators -15DB at the input of the Amp. Didn't help much. I can go to 8 o'clock, very loud. Past that and it's too much. But the audio up to 8 is very clear and sounds great! I stopped the CD play, cranked the volume, and there is no background hum or buzz. Dead silent!
This setup is not what I'm going to use the Preamp with. I was just testing to see how it sounded. I was going to build a EL 34 single end tube amp for a project. I'll wait til I build the Tube Amp for further testing.
I'll follow this thread to see how it works for you MKG.
Please keep us posted.
Michael
P.S. I had wired my transformer for 220 use here in Germany.
This setup is not what I'm going to use the Preamp with. I was just testing to see how it sounded. I was going to build a EL 34 single end tube amp for a project. I'll wait til I build the Tube Amp for further testing.
I'll follow this thread to see how it works for you MKG.
Please keep us posted.
Michael
P.S. I had wired my transformer for 220 use here in Germany.
Attachments
Last edited:
Autobahn
Active Member
Here are the advertised Specs:
- Frequency responses: 10Hz - 100KHz +/- 0dB; 10HZ-200KHZ + -0.2dB
- RMS: 0.8W * 2
- Power supply: AC 220V-240V
- Tubes used: 2pcs 6P6P, 2pcs 6J4, 1pc 653P
- Input: 4ways RCA jacks
- Output: 1way RCA jacks
- Supply votage: All Voltages are available 60Hz / 50Hz
- The machine output voltage of 12V, an enlarged eight times
- Power Transformers 76 * 40 270v * 2 6.3v 3.15v * 2
- Frequency bandwidth of this machine is: 10Hz-100KHz (± 0dB)、10Hz-200KHz (± 0.2dB)
- Size: 29*17*5CM,
- Weight: 6.3 kg
Autobahn
Active Member
The Rothwell Audio -15DB in-line signal attenuators didn't help much with the CD player, but they worked really well with a Technics Cassette tape deck I just tried. I was able to smoothly move volume up to the 12 o'clock postion. Could have probaly went alittle higher, but it's Sunday (neighbors) 
Michael
Michael
I think it would be good to measure the gain of your preamp. Typical preamps have a gain factor of 8x to 10x.
15 dB attenuation in the line attenuators is the same as a gain reduction factor of about 5.5x.
If your preamp is amplifying at a gain factor of 10x, and the attenuators are reducing gain at a factor of 5.5x, that's a forward voltage gain of a mere 1.8x which isn't a lot.
I just wonder if either a) you'd be better off with a passive volume control for your amps, if your amps are super sensitive, or b) your preamp is putting out a lot more gain than is typical . For example is the feedback level set appropriately? Feedback for this preamp is provided by the 24K resistor that connects from the output of the 6V6 into the cathode of the 6AU6.
Anyway if you have the means I think it would be a good next step to measure forward gain on your preamp.
15 dB attenuation in the line attenuators is the same as a gain reduction factor of about 5.5x.
If your preamp is amplifying at a gain factor of 10x, and the attenuators are reducing gain at a factor of 5.5x, that's a forward voltage gain of a mere 1.8x which isn't a lot.
I just wonder if either a) you'd be better off with a passive volume control for your amps, if your amps are super sensitive, or b) your preamp is putting out a lot more gain than is typical . For example is the feedback level set appropriately? Feedback for this preamp is provided by the 24K resistor that connects from the output of the 6V6 into the cathode of the 6AU6.
Anyway if you have the means I think it would be a good next step to measure forward gain on your preamp.
MKG
Elvish-mark’d abortive, rooting hog
1000hz input, laptop to schiit modi 3:
1.952v rms
Preamp output at full volume:
16v rms
So, roughly 8x gain?
As someone above said, I’m better off with a passive attenuator- ridiculously simple to build so I’ll go ahead and do that.
So that I can put this preamp to use at a future date, what type of sources would need 8x gain? Or is it more dependent on the max input voltage of my amp? All of my sources put out significantly more than the max input voltage needed for any of my amps.
1.952v rms
Preamp output at full volume:
16v rms
So, roughly 8x gain?
As someone above said, I’m better off with a passive attenuator- ridiculously simple to build so I’ll go ahead and do that.
So that I can put this preamp to use at a future date, what type of sources would need 8x gain? Or is it more dependent on the max input voltage of my amp? All of my sources put out significantly more than the max input voltage needed for any of my amps.
BinaryMike
Pelagic EE
Gain has to be measured at an output level well below maximum, so your measurement is correct only if 16Vrms output is within spec for that model. I generally conduct gain measurements by setting input level to get some arbitrary output voltage that's well within any preamp's output spec, e.g. 3.00Vrms, then measure input voltage and do the math.
Most preamps have excess gain, so the the usual volume control setting will be under 50% of mechanical span. You will occasionally run into sources such as vintage 150mV Japanese tuners and tape decks, or line/earphone outputs of modern digital sources, that need every bit of 8X~10X preamp gain to drive some power amps to full output. I built a line amp with 10X gain recently for just such requirements. See here: http://audiokarma.org/forums/index.php?threads/cenoté-a-simple-6au6-line-amplifier.831983/
Most preamps have excess gain, so the the usual volume control setting will be under 50% of mechanical span. You will occasionally run into sources such as vintage 150mV Japanese tuners and tape decks, or line/earphone outputs of modern digital sources, that need every bit of 8X~10X preamp gain to drive some power amps to full output. I built a line amp with 10X gain recently for just such requirements. See here: http://audiokarma.org/forums/index.php?threads/cenoté-a-simple-6au6-line-amplifier.831983/
The Hafler DH-200 spec sheet (http://hafler.com/pdf/archive/DH-220_amp_man.pdf, page 16) says input sensitivity is 1.55V input for 115 watts output. If you normally listen at say 20 watts output, for average SPL, you won't need much input voltage at all to achieve that. So it does seem your active preamp is overkill for this application.
By way of reference, a standard digital source puts out 2V, so it sure does seem to me something like a quick and dirty 50K passive attenuator would do the job nicely.
By way of reference, a standard digital source puts out 2V, so it sure does seem to me something like a quick and dirty 50K passive attenuator would do the job nicely.
I think you can make your preamp work for your current setup if you don't mind rewiring a few things. Might as well try to make it work, right? Otherwise it will likely sit on the shelf not being used for a while. Anyway, take a look at this reworked schematic:
This lets you use much of your existing power supply and 6AU6 stage while retaining the octal tube socket where the 6V6 was. The big differences between your original schematic and this one are:
You can always add a bit of feedback later into the circuit if you want. I'd do that by replacing the 47K resistor in the inter-stage voltage divider (right below the 0.33 uF cap) with a straight wire (this will remove the inter-stage attenuation), then add maybe a 100K resistor from output back to the junction of the 0.33 uF cap and 1.5K resistor on the input to the 6AU6.
Anyway, just an idea that will allow you to experiment without shelving the preamp.
This lets you use much of your existing power supply and 6AU6 stage while retaining the octal tube socket where the 6V6 was. The big differences between your original schematic and this one are:
- The replacement of the 6V6 with an octal small signal triode, the 6J5, which is used as a cathode follower. This will let you drive pretty much any power amp you want. (and to be fair, your original preamp allowed that too).
- The removal of all negative feedback, so more "tube" sound, less "hi-fi" sound, but might now be a bit noisy unless you find some quiet 6AU6s.
- Rebiasing of the first stage for lower gain.
- Addition of some inter-stage attenuation that provides gain reduction by a factor of 0.875x. (to bring overall gain to about half of your original design)
- Will require you to hunt down some 6J5's.
- Pulls one-half of the current from the power supply than the original design did (by replacing the current hungry 6V6 with the small signal 6J5 triode).
- Will invert signal polarity on output, whereas your original preamp design did not invert polarity. (Adjust for it at the speaker terminals of the power amp if you care.)
You can always add a bit of feedback later into the circuit if you want. I'd do that by replacing the 47K resistor in the inter-stage voltage divider (right below the 0.33 uF cap) with a straight wire (this will remove the inter-stage attenuation), then add maybe a 100K resistor from output back to the junction of the 0.33 uF cap and 1.5K resistor on the input to the 6AU6.
Anyway, just an idea that will allow you to experiment without shelving the preamp.
Last edited:
Also I'd suggest other AK members are free to comment on the proposed new design..(if anyone is interested in doing so). Might help bring out any vulnerabilities in the proposed new design.
I can think of one issue I might look at, which is the 6J5 is sitting at 100V above ground, which is getting close to the limit of heater-cathode max voltage. I think it will be okay, but to really make it bullet proof, you could bias up the heater circuit by say 20V DC. (I wouldn't go more than that.)
I can think of one issue I might look at, which is the 6J5 is sitting at 100V above ground, which is getting close to the limit of heater-cathode max voltage. I think it will be okay, but to really make it bullet proof, you could bias up the heater circuit by say 20V DC. (I wouldn't go more than that.)
I made two typos.
The cathode resistance on the 6J5 should be split as 1.3K and 18K resistors. This will bias it at 200V across the tube, approximately 5 mA quiescent current, and about -7V difference between grid and cathode.
The power supply dropping resistor between the two stages should be 12K.
Also MKG, if you could measure the DC voltage on pin 8 of the rectifier that will help hone in a little better on what the power supply resistors should be. I estimated 1.5K each, but they might be a bit high or a bit low. Basically tweak those resistors until you get 300V being delivered to the plate of the 6J5's.
The cathode resistance on the 6J5 should be split as 1.3K and 18K resistors. This will bias it at 200V across the tube, approximately 5 mA quiescent current, and about -7V difference between grid and cathode.
The power supply dropping resistor between the two stages should be 12K.
Also MKG, if you could measure the DC voltage on pin 8 of the rectifier that will help hone in a little better on what the power supply resistors should be. I estimated 1.5K each, but they might be a bit high or a bit low. Basically tweak those resistors until you get 300V being delivered to the plate of the 6J5's.
Last edited:
