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Old 24th March 2009, 07:16 PM   #1
Cambo is offline Cambo  Canada
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Default +4 dBu to -3dBv conversion

I have a technical conundrum and was wondering if somebody out there could help solve it for me. I have an audio device (tube preamp) whose output is the normal +4dBu running into an amplifier (B&O Icepower) whose optimal input level is -3dBv. My preamp can cause severe distortion and clipping in the amp, and the final level control is SO touchy it's ridiculous - is there an easy way to convert a +4dBu output to -3dBv? Is it just resistors and stuff, can I make a little passive box or converter cable, or is it WAY more complicated that that, requiring power, transformers, heat-sinks, etc?

I've been getting the runaround for month on this from the preamp maker and just want to get it solved. Any help would be extremely appreciated.

Cheers,
Cambo
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Old 24th March 2009, 07:37 PM   #2
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It looks like you need about 5dB of attenuation roughly (about 1.2 volts down to 0.7 volts). That is the easy part. But we don't have quite enough information yet (re: input and output impedance of the source and load).

However, the general strategy is to either use a voltage divider which can be in the form of a pair of resistors (nothing fancy). Or it can be in the form of a pot (potentiometer).

The other strategy is to use a transformer to step down the voltage. This strategy will be more expensive.

In either case, remember to consider the issue of impedance of the two devices (source and load)
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Old 24th March 2009, 09:13 PM   #3
fotios is offline fotios  Greece
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Quote:
Originally posted by WithTarragon
It looks like you need about 5dB of attenuation roughly (about 1.2 volts down to 0.7 volts). That is the easy part. But we don't have quite enough information yet (re: input and output impedance of the source and load).
Your rough calculation of: 1,2Vrms = +4dBu (20 X log 1,2Vrms/0,775) and 0,7Vrms = -3dBV (20 X log 0,7Vrms/1) are correct.
The only that i have to add (if you allow it to me) it is that i think the dBu scale was established for using regardless of Zo and Zi. According this, a simple voltage divider with a ratio of 1,4 can do the work... maybe?

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Fotios
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Old 24th March 2009, 09:56 PM   #4
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I wonder if an attenuation of around 5dB will make any BIG difference. Its quite a small change in perceved level, and if you are getting SEVERE distortion, that degree of attenuation won't change much.

However, if you used a potentiometer as a divider, you could adjust it on the fly, and find the optimum by trial and error.
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Old 24th March 2009, 09:59 PM   #5
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Fotios, We are in agreement. The dBu and dBV scales are both 20 log ratios of voltage. Since we are not considering power, the impedance (Z) does not directly enter into it.

However, this is my thinking of why I brought up the issue of Z out of the "source" and Z in of the "load". If this were a source (say a CD player) going to an amplifier (load) and you put a potentiometer in between (our voltage divider). You would then worry about having a large impedance mis-match (low to high) between the source and the pot and also between the pot and the load (again, low to high). Basically you are trying to transfer voltage not power. By ensuring the impedance mis-match (low to high) you will not "draw" current from the source (which might not be able to provide much current.

Is my thinking clearer? Is it an issue in this case?
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Old 24th March 2009, 10:05 PM   #6
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Default Re: +4 dBu to -3dBv conversion

Quote:
Originally posted by Cambo
I have a technical conundrum and was wondering if somebody out there could help solve it for me. I have an audio device (tube preamp) whose output is the normal +4dBu running into an amplifier (B&O Icepower) whose optimal input level is -3dBv. My preamp can cause severe distortion and clipping in the amp, and the final level control is SO touchy it's ridiculous - is there an easy way to convert a +4dBu output to -3dBv? Is it just resistors and stuff, can I make a little passive box or converter cable, or is it WAY more complicated that that, requiring power, transformers, heat-sinks, etc?

I've been getting the runaround for month on this from the preamp maker and just want to get it solved. Any help would be extremely appreciated.

Cheers,
Cambo
Make your preamp passive, it will probally sound better (more transparent) without the colourations of the tubes and it won't cost you a penny, just 30mins or so in time.

Disconnect the input tube from the volume pot and the output from the output rca's.
And then take the output of the volume control that you disconected from the input tube and take it to the output rca's and bypass the tube section.
It sounds like you will still have enough gain, even in passive mode.
Added bonus is you won't have to the power it up to use it. (Green Preamp)


Cheers George
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Old 24th March 2009, 10:26 PM   #7
Cambo is offline Cambo  Canada
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Default Thanks so far...

This is from the preamp makers website:

Output: +4 dBu corresponds to 0 VU. The output is electronically balanced or unbalanced. Output impedance is 75 ohms. The recommended output load is 600 ohms or more. Maximum input level is +19 dB

From the Amp manual:

The input sensitivity on the amp is .70 volts (-3 dbv) for full power. Input impedance is 39K ohms.

I am running the system balanced with xlr connectors.

Thanks so much. What I need is some simple plans for a converter - this is quite a high-end electric bass system, and I want something that sounds really good, or I should say does not detract from the sound or adds any noise or colour of it's own - it is all in a rack so I don't mind a transformer in a box or something like that, although simpler would be better.

Cheers,
Cambo
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Old 24th March 2009, 10:51 PM   #8
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Which amp model and which preamp model?
Cheers George
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Old 24th March 2009, 11:29 PM   #9
fotios is offline fotios  Greece
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Quote:
Originally posted by WithTarragon
Fotios, We are in agreement. The dBu and dBV scales are both 20 log ratios of voltage. Since we are not considering power, the impedance (Z) does not directly enter into it.

However, this is my thinking of why I brought up the issue of Z out of the "source" and Z in of the "load". If this were a source (say a CD player) going to an amplifier (load) and you put a potentiometer in between (our voltage divider). You would then worry about having a large impedance mis-match (low to high) between the source and the pot and also between the pot and the load (again, low to high). Basically you are trying to transfer voltage not power. By ensuring the impedance mis-match (low to high) you will not "draw" current from the source (which might not be able to provide much current.

Is my thinking clearer? Is it an issue in this case?
Your assumptions, are correct generally.
Very glad to meet you!
From a second view, i suppose that Cambo case of level mismatching it is not a serrious problem. It is very common in most systems. For example, in my projects i don't care anymore for I/O levels so much. In my power amplifiers i use a Zin of 50K (unbalanced) and 15K (balanced) just only from habbit. I can play with the Av given from the feedback loop resistors ratio, to obtain an input sensitivity between 0,775Vrms (0 dBu) to 1,414Vrms (+3dBV). And this does no matters... Because in my discrette implemented preamplifiers, the output buffers (balanced or unbalanced) incorporate as output devices pairs of D44/45H11 which can drive simultaneously except one or two power amplifiers as well a pair of headphones of Z=24. There is no problem of overloading. Simply in my power amplifiers i place allways a pair of clipping indicators to feel safe from overloadings.
I don't know if Cambo's devices have any indication instrument. If yes, then i think there is no reason for intervention inside electronic circuits for modifications. Just a little care is enough.

Regs
Fotios
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Old 24th March 2009, 11:48 PM   #10
fotios is offline fotios  Greece
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Quote:
Originally posted by georgehifi
Which amp model and which preamp model?
Cheers George
OUT OF TOPIC!
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What do you think about Casey Stoner?

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