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
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
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)
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)
WithTarragon said:
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?
Regs
Fotios
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.
However, if you used a potentiometer as a divider, you could adjust it on the fly, and find the optimum by trial and error.
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?
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?
Cambo said:
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
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
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
WithTarragon said:
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 50KÙ (unbalanced) and 15KÙ (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=24Ù. 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
georgehifi said:
OUT OF TOPIC!
Forgive me please, but Mick Doohan was and will be my beloved rider forever. I can't forget never his amazing overtakes in the last lap (usually) many times on two bikes simultaneously!
Mick was except an artist on two wheels as well a brave-heart.
What do you think about Casey Stoner?
Fotios
Re: Re: +4 dBu to -3dBv conversion
Thanks, but no thanks. They are Summit Audio pieces, the 2BA Preamp and the TLA 50 compressor, so I'd rather not do any internal modifications, and I certainly don't want to remove the tube...
the last volume control on the TLA 50 is SO touchy you wouldn't believe it - but I need to convert the input level going to the amp.
And the 5db number does NOT sound right - I think it's more like 20 db of overload.
Thanks,
Cameron
georgehifi said:
Thanks, but no thanks. They are Summit Audio pieces, the 2BA Preamp and the TLA 50 compressor, so I'd rather not do any internal modifications, and I certainly don't want to remove the tube...
the last volume control on the TLA 50 is SO touchy you wouldn't believe it - but I need to convert the input level going to the amp.
And the 5db number does NOT sound right - I think it's more like 20 db of overload.
Thanks,
Cameron
Hi Cameron
Please don't missunderstand me... but here, in "solid state forum" you can't find answeers in your questions, because this forum is addressed mainly for discussions about Hi-Fi components for music reproduction. All replies posted, are under the assumption that you ask for such type devices, thus you can understand that you deceived us.
Your last post reveals that your thread is related with some musical instruments proccesing devices. Because you are musician, at least me i can't offer any help to you. You know your issue better from all of us because... you are the performer! Please, moove your thread in "Musical Instruments Forum", and i believe that overthere you will find more concrete proposals; from expertised members which are performers like you.
Regs
Fotios
Please don't missunderstand me... but here, in "solid state forum" you can't find answeers in your questions, because this forum is addressed mainly for discussions about Hi-Fi components for music reproduction. All replies posted, are under the assumption that you ask for such type devices, thus you can understand that you deceived us.
Your last post reveals that your thread is related with some musical instruments proccesing devices. Because you are musician, at least me i can't offer any help to you. You know your issue better from all of us because... you are the performer! Please, moove your thread in "Musical Instruments Forum", and i believe that overthere you will find more concrete proposals; from expertised members which are performers like you.
Regs
Fotios
Hi,
can you measure the AC output voltage of the preamp?
Have you checked the DC voltage at the output of the preamp?
I suspect the 700mVac sensitivity is about right.
It appears it's your beloved preamp that is causing your problem.
Is the ground connection of the output attenuator broken? That will create a weird law on the output attenuation.
can you measure the AC output voltage of the preamp?
Have you checked the DC voltage at the output of the preamp?
I suspect the 700mVac sensitivity is about right.
It appears it's your beloved preamp that is causing your problem.
Is the ground connection of the output attenuator broken? That will create a weird law on the output attenuation.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.