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Setup Question (do I really need the gain from a preamp)

My setup consists of a MM cartridge that has a nominal output of 5.5mV, an op-amp based phono stage with a gain of 40dB (A=100 V/V), a 10K Alps volume control that runs into a NAD 214 power amp (80 WPC). The power amp has an input sensitivity of 0.9V and an input impedance of 68K/500pF. I have a pair of Tannoy Mercury V4 floor-standing speakers which have a sensitivity of 91dB/m.

Generally I run my volume control about halfway up and it is plenty loud. I have been thinking of building a tube preamp for a while. I could then integrate my standalone volume control into the preamp and also get another ~10dB of gain. A lot of the reason I would like the preamp though is I like to build and design... :nod:

What I am wondering, is it worth it, or does it make sense to add a preamp to my system? With my current setup the 0.0055V signal from the turntable can be amplified to 0.55V (0.0055V*100 from phono stage). So I’m only driving my power amp to 61% of its rated sensitivity. But at the same time the volume is ok for me.

Is there something I am missing here or am I ok without a preamp? Do I need that extra gain? Is there anything a preamp will do sound wise besides being able to drive my power amp to the fullest extent?

Thanks!
 

rayma

Member
2011-04-29 8:37 pm
Do I need that extra gain? Is there anything a preamp will do sound wise besides
being able to drive my power amp to the fullest extent?

Remember that you need headroom above the average 5.5mV phono input level.
With your system gain, music peaks can easily drive the NAD to full output.
If you have long audio cables to the amp, you could try a buffer after the volume control.
 
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Ray is right about buffered volume controls frequently being all that's needed. NAD power amps are known for having good dynamic headroom. So a tubed preamp with a little gain might work out nicely for you, when playing LPs. For "standard" 2 VRMS digital sources you would need to use padding resistors, else a "hair trigger" volume control will occur.

Quite a few low gain 12B4 based designs are available. FWIW, I've uploaded mine. The fellow who made that drawing from a narrative I provided has a marked penchant for "boutique" parts. An excellent implementation of the concept can be done with good grade industrial parts and costs contained.
 

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Remember that you need headroom above the average 5.5mV phono input level.
With your system gain, music peaks can easily drive the NAD to full output.
If you have long audio cables to the amp, you could try a buffer after the volume control.
The 5.5mV is quoted at 1000 Hz, 5cm/sec.: 5.5 mV. What do cartridges peak at generally? I want to get a sense of what is coming out of my cartridge and how close I am coming to driving my amp to full power.


Ray is right about buffered volume controls frequently being all that's needed. NAD power amps are known for having good dynamic headroom. So a tubed preamp with a little gain might work out nicely for you, when playing LPs. For "standard" 2 VRMS digital sources you would need to use padding resistors, else a "hair trigger" volume control will occur.

Quite a few low gain 12B4 based designs are available. FWIW, I've uploaded mine. The fellow who made that drawing from a narrative I provided has a marked penchant for "boutique" parts. An excellent implementation of the concept can be done with good grade industrial parts and costs contained.
I have heard lots of talk about the 12B4. I have been researching the Akidio linestage a lot, I like the low output impedance and PSU rejection. Has anyone done a 12B4 Akidio? Are the 12B4 preamp designs usually just common cathode single gain stages?
 
I have heard lots of talk about the 12B4. I have been researching the Akidio linestage a lot, I like the low output impedance and PSU rejection. Has anyone done a 12B4 Akidio? Are the 12B4 preamp designs usually just common cathode single gain stages?

Most, if not all, of the 12B4 designs are common cathode. The schematic I uploaded employs a well regulated pseudo choke I/P filter PSU and constant current source (CCS) loading. PSRR is truly a non-issue. The circuit I uploaded can drive the awkward 10 Kohm IHF "standard" load. The 12B4 is 1 of the few tubes wired common cathode that can.

I think the otherwise excellent Aikido setup has too much gain and the "hair trigger" volume control boogieman will plague you.
 
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Jay

Banned
2003-02-11 9:02 am
Jakarta
Is there something I am missing here or am I ok without a preamp? Do I need that extra gain? Is there anything a preamp will do sound wise besides being able to drive my power amp to the fullest extent?

Distortion. One transistor may be able to produce sufficient gain, but three transistors doing the same thing might produce lower total distortion. This, of course, requires that you PLAN the gain structure along the chain (not just plug and play).

As a simple example, a gain stage may produce a certain distortion driving low impedance. An additional buffer stage will increase the impedance and lower the previous gain stage distortion.

Here is a more complex example, assume this is a complete chain, you have one opamp driving volume pot and then another opamp after the pot, with circuit gain of 10dB. How would you set the gain distribution? This really depends on several things including the opamps being used (assuming no issue with source and load impedance).

With preamp addition, the cost is usually with intolerable increase of noise, so make sure you use better than average power supply for the preamp, because it is usually the bottleneck.

If you use opamps for the preamp, you're in trouble, because JFET-input opamps are very musical but the noise is audible and almost unacceptable in all designs I've tried. A surprise for me that a bipolar opamp like JRC2068 has lower noise than NE5532 but musically it is even not worse than JFET opamps.

What opamp do you use for your source? I found that 4562 sounds superior with higher gain (such as probably your Av=100).
 
@Eli Duttman,

Ooops. Thank you, HeyBill

If you want to use the pseudo choke I/P filter PSU shown, I can help you select "iron" that's reasonably priced.

BTW, the exact value of the "fudge factor" cap. has be determined at the bench. A value that get's the rail above 120 V., but not more than 125 V. is appropriate. Regulation suffers and critical current behavior disappears, somewhere around 1 μF. :( It becomes a cap. I/P filter.
 

20to20

Member
2010-06-23 9:25 pm
The power amp has an input sensitivity of 0.9V and an input impedance of 68K/500pF.

a 10K Alps volume control that runs into a NAD 214 power amp

I could then integrate my standalone volume control

Is there something I am missing here or am I ok without a preamp? Do I need that extra gain? Is there anything a preamp will do sound wise besides being able to drive my power amp to the fullest extent?

Thanks!

Your 10K pot is in parallel with the input so the true input impedance is more like 8K total. I'd take that up to a 50K or 100K pot first and see if that gives you what you're lacking... if anything but volume selectivity.
 
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Your 10K pot is in parallel with the input so the true input impedance is more like 8K total. I'd take that up to a 50K or 100K pot first and see if that gives you what you're lacking... if anything but volume selectivity.

The 10 Kohm pot. is, for practical purposes, the load presented to the upstream source(s), as the power amp's I/P impedance is in parallel with that portion of the control's resistance "below" the wiper. The OP indicated an opamp based phono preamp, which should be fine working into 10 Kohms, which happens to be the IHF "standard" load.

However, the pot. needs to work into a load that is greater than 10 KOhms, ideally 100 Kohms or more. The $64 question is about the NAD power amp's I/P impedance. Few SS power amps are good candidates for use with passive control centers "taller" than 10 Kohms. Due to a low I/P impedance, many SS power amps mate very poorly with passive control centers. Buffering the volume pot. eliminates any possible impedance mismatch headache both upstream and downstream, while not adding potentially undesirable voltage gain.
 

20to20

Member
2010-06-23 9:25 pm
The input sensitivity of the power amp becomes the same as if you had an 8K input impedance to start with there. The op amp may be just fine with 8K to drive but the power amp isn't going to see much signal. The op amp is going to give .5v max when the sound in the record is hammering loud out of the cartridge. So the power amp needs more signal if the input impedance is cut to 8K max.
 
Distortion. One transistor may be able to produce sufficient gain, but three transistors doing the same thing might produce lower total distortion. This, of course, requires that you PLAN the gain structure along the chain (not just plug and play).

As a simple example, a gain stage may produce a certain distortion driving low impedance. An additional buffer stage will increase the impedance and lower the previous gain stage distortion.

Here is a more complex example, assume this is a complete chain, you have one opamp driving volume pot and then another opamp after the pot, with circuit gain of 10dB. How would you set the gain distribution? This really depends on several things including the opamps being used (assuming no issue with source and load impedance).

With preamp addition, the cost is usually with intolerable increase of noise, so make sure you use better than average power supply for the preamp, because it is usually the bottleneck.

If you use opamps for the preamp, you're in trouble, because JFET-input opamps are very musical but the noise is audible and almost unacceptable in all designs I've tried. A surprise for me that a bipolar opamp like JRC2068 has lower noise than NE5532 but musically it is even not worse than JFET opamps.

What opamp do you use for your source? I found that 4562 sounds superior with higher gain (such as probably your Av=100).
I am using OPA2134. It is a FET based op-amp.

I will have to try the LM4562 sometime.

Which Fig. 1-3, there is many?
What do "most" records peak at?
 
The 10 Kohm pot. is, for practical purposes, the load presented to the upstream source(s), as the power amp's I/P impedance is in parallel with that portion of the control's resistance "below" the wiper. The OP indicated an opamp based phono preamp, which should be fine working into 10 Kohms, which happens to be the IHF "standard" load.

However, the pot. needs to work into a load that is greater than 10 KOhms, ideally 100 Kohms or more. The $64 question is about the NAD power amp's I/P impedance. Few SS power amps are good candidates for use with passive control centers "taller" than 10 Kohms. Due to a low I/P impedance, many SS power amps mate very poorly with passive control centers. Buffering the volume pot. eliminates any possible impedance mismatch headache both upstream and downstream, while not adding potentially undesirable voltage gain.
Hi Eli

Thanks for the information and clarification. The op-amp based phono stage can easily drive the 10K pot as mentioned. The issue is there is nowhere to put the volume pot besides between the phono and the poweramp currently.

If I did build a little 12B4 preamp I could but the volume control before the preamp (and thus poweramp). The phono stage could easily drive the 10K pot and following preamp stage would provide some gain with a low output impedance to the poweramp (68K input impedance).

I think I will research the 12B4 stage some more. I might message you with some questions later if that's ok.
 
Shure's recording survey found ~60cm/sec peak recorded velocity.
That corresponds to about 42cm/sec rms, or +18.5dB more than your cartridge's rated rms velocity.
Interesting, I see it now!

18.5dB corresponds to an Av of 8.4. So my nominal 5.5mV is sometimes peaking at 0.046V, which after the phono stage is 4.6V.

I have never experienced any clipping though...
 

rayma

Member
2011-04-29 8:37 pm
18.5dB corresponds to an Av of 8.4. So my nominal 5.5mV is sometimes peaking at 0.046V,
which after the phono stage is 4.6V. I have never experienced any clipping though...

I suspect that most cartridges would mistrack before that peak level. Your preamp (and most using op amps)
should be able to output around 10Vrms with the usual +/-15V supplies.
 
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