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Old 2nd January 2004, 07:59 PM   #1
Ralph is offline Ralph  Netherlands
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Default Measurements of tube amps

Recently I've built a new amplifier, a 300B XLS one. This was my first tube amp coming from a Linn solid state one. Now I'm not getting high enough sound levels in this new setup when playing 'quiet' cd's. I'm used to play cd's on a moderate level (not too loud).

I can imagine 4 things where I can have a mismatch. Ofcouse they are all related.

1/ sensitivity of my loudspeakers. They are not very sensitive (around 87dB/2.83V/1m). I am concidering building new ones.

2/ output of my cdp. This is 2.6V for 0dB and that looks OK.

3/ voltage gain of my pre-amplifier. I measured this to be 15dB. I really don't know if that's low, high or enough.

4/ input sensitivity of the 300B amp. schematics I guess the input sensitivity for full output is somewhere in the 4.5V area.

Can you comment on 3/ and 4/ please? If a CD puts out 0 dB, I reach the input sensitivity of the amplifier with the pre-amp on 18 (out of 23). But when a CD puts out -20dB (an average CD?) the pre-amp puts out a maximum of 1.45V and never drives the amplifier to full potential levels.

So, is it OK to say that either my pre-amp has not enough voltage gain or my poweramp is not sensitive enough?

Or do the pre-amp and amp match OK and can I solve 'the problem' by using more sensitive speakers (let's say around 91 dB/2.83V/1m)?

Ralph
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Old 2nd January 2004, 09:35 PM   #2
dhaen is offline dhaen  Europe
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Hi Ralph,

I think that your amp should have a gain of around 10dB. This is on the low side, but no problem in its self.
You should be able to check this yourself.
measure V in and V out. Use a sinewave tone.
The gain will be: 20Log (V out / V in)

Raising the gain of the amp is not trivial. It would mean changing the driver valve or driver transformer. Perhaps someone knows of a plug-n replacement for the 5687WA with a bit (a lot) more gain. A higher gain replacement is unlikely to sound as sweet.

My preference would be to use a step-up signal transformer between the pre-amp and the amp. It would depend on the preamp's output impedance though.

Another idea is to see if the pre-amp is easy to modify.
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Old 2nd January 2004, 09:54 PM   #3
316a is offline 316a  England
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Default Re: Measurements of tube amps

Quote:
Originally posted by Ralph
Recently I've built a new amplifier, a 300B XLS one. This was my first tube amp coming from a Linn solid state one. Now I'm not getting high enough sound levels in this new setup when playing 'quiet' cd's. I'm used to play cd's on a moderate level (not too loud).

I can imagine 4 things where I can have a mismatch. Ofcouse they are all related.

1/ sensitivity of my loudspeakers. They are not very sensitive (around 87dB/2.83V/1m). I am concidering building new ones.

2/ output of my cdp. This is 2.6V for 0dB and that looks OK.

3/ voltage gain of my pre-amplifier. I measured this to be 15dB. I really don't know if that's low, high or enough.

4/ input sensitivity of the 300B amp. schematics I guess the input sensitivity for full output is somewhere in the 4.5V area.

Can you comment on 3/ and 4/ please? If a CD puts out 0 dB, I reach the input sensitivity of the amplifier with the pre-amp on 18 (out of 23). But when a CD puts out -20dB (an average CD?) the pre-amp puts out a maximum of 1.45V and never drives the amplifier to full potential levels.

So, is it OK to say that either my pre-amp has not enough voltage gain or my poweramp is not sensitive enough?

Or do the pre-amp and amp match OK and can I solve 'the problem' by using more sensitive speakers (let's say around 91 dB/2.83V/1m)?

Ralph
Hello ,
The 5687 could be replaced with a pair of higher mu and similar Ra single triodes . Not too many choices though but EC8010 springs to mind with 417a/5842 a close second . Both of these are linear and should increase the gain somewhat but you still may not have enough gain for quiet CD's .

316a
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Old 3rd January 2004, 08:26 AM   #4
Ralph is offline Ralph  Netherlands
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Quote:
Not too many choices though but EC8010 springs to mind with 417a/5842 a close second .
What I forgot to mention is that I can only use one 5687 driver tube in my amplifier, you see the little one on the right.

Quote:
Another idea is to see if the pre-amp is easy to modify.
Yes, it probably is. It uses 5:1 stepdown transformers so I could replace them or use a different drivertube (now E188CC). Will think about that a bit longer.

Thanks, Ralph
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Old 3rd January 2004, 08:57 AM   #5
EC8010 is offline EC8010  United Kingdom
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Default Re: Measurements of tube amps

Quote:
Originally posted by Ralph
But when a CD puts out -20dB (an average CD?) the pre-amp puts out a maximum of 1.45V and never drives the amplifier to full potential levels.

The very first CDs didn't hit 0dBFS because the metering at that time was so bad that mastering engineers couldn't be certain of avoiding digital clipping, so they played safe and mastered at a lower level. Modern CDs are digitally conformed so that the loudest peak nudges 0dBFS. Modern pop recordings have so much compression that that they are close to 0dBFS all the time (Sheryl Crow is a case in point).

A few quick calculations suggested that the sensitivity of your amplifier is just about 2VRMS for full power. So if your pre-amplifier has a gain of 15dB, you are easily able to drive your 300B amplifier to full power on any CD. If it isn't loud enough, then you need more power, or more efficient loudspeakers.
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Old 3rd January 2004, 09:14 AM   #6
Ralph is offline Ralph  Netherlands
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Quote:
A few quick calculations suggested that the sensitivity of your amplifier is just about 2VRMS for full power.
Sensitivity of my amplifier is around 4.12V. Bias is -95V (-68VRMS) and mu of 5687 is 16.5. 68/16.5=4.12V.

But I agree with you that most modern poprecordings use so little dynamics that they are constantly in last 10dB area.
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Old 3rd January 2004, 09:29 AM   #7
EC8010 is offline EC8010  United Kingdom
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Whoops! I calculated assuming that you had already used a 5842.
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Old 3rd January 2004, 10:46 AM   #8
Ralph is offline Ralph  Netherlands
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I think in the end it is just a matter of configuration. I did some tests by setting the pre-amp on click 17 (of 23), that is around 3 o'clock. On this level the pre-amp puts out a max of 3.6V when the CD hits 0dB. So we can be sure the amplifier never clips and almost reaches it full output.

Than I played some well known pop CD's and measured the average level with CoolEdit Pro.

1/ Toto - Africa (average RMS power -22.19 dB, max RMS power -11.79 dB)

Sounds moderate/loud enough for me.

2/ Tim Finn - Fraction too much friction (avg RMS -19.09 dB, max RMS -8.89 dB)

Sound louder, a little too loud for my taste.

3/ Dire Straits - Sultans of swing (avg RMS -17.02 dB, max RMS -8.24 dB)

Definitly louder, too loud for me.

4/ Madonna - The power of goodbye (avg RMS -12.65 dB, max RMS -6.49 dB)

Definitly louder, too loud for me.

So maybe I can draw the conclusion that my loudspeakers are sensitive enough for this amp because without overloading the amp, I get loud enough levels.`

It's just that with this combination of pre and power, I have (only) 6 volumelevels left on the pre-amp to boost quieter CD.
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Old 3rd January 2004, 10:56 AM   #9
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Hi,

Quote:
It's just that with this combination of pre and power, I have (only) 6 volumelevels left on the pre-amp to boost quieter CD.
The main question is how many dBs each of the remaining steps cuts out.

If each cuts at 3dB, you'll still have 18dB of amplification left which is still alot.

Cheers,
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Old 3rd January 2004, 11:21 AM   #10
Ralph is offline Ralph  Netherlands
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It's about 2dB per step

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