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Old 8th August 2015, 11:25 AM   #1
oreo382 is offline oreo382  Canada
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Default Technical advice needed

This is my first visit to this forum. I have a question I hope someone can answer for me. I have built a Pass Aleph J power amp that requires 1.5 volts for full output. I am currently feeding it from my cable tv pvr which has variable analog rca line level outputs with a max level of 1 volt.It also has a set of fixed 1 volt rca outputs as well. I have fairly inefficient speakers ( New Large Advents) and would like to be able to drive them fully.
Would a dac with volume pot work or would I need a preamp? If either would work should I connect to the variable or fixed output?
Thank you in advance for any advice I receive.
BTW the Pass amp is incredible, makes the Advents sing!
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Old 8th August 2015, 11:34 AM   #2
AndrewT is offline AndrewT  Scotland
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You need a little bit of gain. see last para.
Do you have any other sources?
What maximum signals do they give out?

In general it is better for performance if each sources gets just enough gain and no more.

This type of control philosophy can be met with a switchable gain pre-amplifier.

The loudest source uses the straight through Buffer setting where added gain is +0dB.
then have a switch that increases the gain by around 5 or 6db for medium level sources and a further gain setting of +10 to +12dB for the lowest output sources.
If your mood suits lower level listening "now" then even the medium and low level sources can be used with the +0dB gain setting. You add the gain when your mood demands louder listening.

BTW, the difference between 1.5Vac and 1Vac is 3.52dB
Theoretically that is all the extra gain that your pre-amp needs for this source.
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Last edited by AndrewT; 8th August 2015 at 11:37 AM.
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Old 8th August 2015, 11:47 AM   #3
oreo382 is offline oreo382  Canada
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Wow, that was quick, thanks Andrew. To answer your questions as I understand them the only source I have is the audio channels from my cable tv. This is fed thru my cable tv box (pvr) which has a fixed 1 volt output and also a variable ( 0-1volt) output). Both these outputs are via rca stereo jacks. I am feeding this into my Aleph J power amp.
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Old 9th August 2015, 08:06 AM   #4
AndrewT is offline AndrewT  Scotland
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Since you only have one source, then a fixed gain pre-amp would do the job of taking your 1Vac upto the max power sensitivity of 1.5Vac.

How often you you find that you have to turn your existing volume control past halfway?
and how often all the way to the top?
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Old 9th August 2015, 02:05 PM   #5
oreo382 is offline oreo382  Canada
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Andrew,
I can see where you are going with this and I may have been wrong as far as stating 1 volt is the output level of my source. I copied this below from Wikipedia. It talks about normal line out levels for consumer and pro audio equipment. My PVR is a Motorola DCT3416 so I am assuming it would be considered consumer. I have looked online and can't get any info on the RCA output levels of this unit. So looking at what is stated the nominal is 0.316 VRMS ? Maybe you can interpret this because I can't. Although listening at max level is fairly loud, not room shaking or ear splitting.




The most common nominal level for consumer audio equipment is −10 dBV, and the most common nominal level for professional equipment is +4 dBu (by convention, decibel values are written with an explicit sign symbol).
Expressed in absolute terms, a signal at −10 dBV is equivalent to a sine wave signal with a peak amplitude (VPK) of approximately 0.447 volts, or any general signal at 0.316 volts root mean square (VRMS). A signal at +4 dBu is equivalent to a sine wave signal with a peak amplitude of approximately 1.736 volts, or any general signal at approximately 1.228 VRMS.
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Old 9th August 2015, 06:57 PM   #6
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It is unfortunate that maximum audio output for the PVR is not specified, as that would allow estimating typical levels. I would assume 0 dBFS level is somewhere between about 1 and 2 Vrms though. Average broadcast levels can be quite low compared to what you find on CDs these days, for example EBU R.128 is shooting for the equivalent of -18 dBFS of pink noise (resulting in 0.25 Vrms on a 2 Vrms output).

Considering this, you may find that you can make use of up to 10 dB worth of gain. A decent bog standard pre of 16.5 dB at reasonable noise levels may well get the job done. Since you have a variable output, even a fixed gain setup may do. The specifics would depend on what's easiest for you, from building a cMoy-ish battery-powered op-amp circuit to buying a (used) hi-fi pre.

Some research reveals that the speakers range at about 88 dB / W / m. Depending on listening distance and surroundings, I wouldn't expect them to get to party levels with the 25 wpc Aleph J, but quite loud nonetheless.
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Old 9th August 2015, 09:31 PM   #7
oreo382 is offline oreo382  Canada
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Thanks for the input so far from you guys. I did some more reading and the Wiki says "The reference voltage for the decibel unloaded (0 dBu) is the AC voltage required to produce 1 mW of power across a 600 Ω load (approximately 0.7746 VRMS).[2] This awkward unit is a holdover from the early telephone standards, which used 600 Ω sources and loads, and measured dBm dissipated in them. Modern audio equipment does not use 600 Ω matched loads, hence the use of "dBm unloaded" = dBu."

This value of 0.7746 V would give me about 8 watts with the Aleph's gain of 20db, which figures to about 97 db given the 88db speaker sensitivity. I think my calculations are accurate. This makes sense given the loudness I am hearing. The Aleph is so clean that the level doesn't sound too loud.
Thanks again for your input, DIY members are a great bunch, always willing to help.
Dave
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