Ascertaining passive preamp output voltage

[wizons]

I'm using a TVC passive preamp to attenuate the analogue signal between my DAC and headphone amp. This might seem odd, but the unbalanced output voltage of the DAC is 4Vrms (sic) and the input sensitivity of the headphone preamp is 300mV. I would imagine that the headphone input would be saturated with such a high voltage signal.

The DAC's analogue output does have (digital) attenuation, but I prefer to keep the output as high as possible so as not to compromise the signal.

My question is this. Without using a multimeter, is it possible to estimate the voltage put out by the TVC preamp for any level of attenuation applied? The attenuator on the TVC preamp moves through 24 steps, from absolute attenuation, then 52dB of attenuation, 50dB and so on up to 0dB of attenuation.

If it is possible to estimate where the attenuator should be set on the TVC so as to allow a maximum signal voltage of 2V to be passed to the headphone amp, I imagine matters would be improved. Common sense suggests it should be set half way, but I know that these things don't necessarily work in a common sense manner!

Many thanks,

 

[SergeAuckland]

I'm using a TVC passive preamp to attenuate the analogue signal between my DAC and headphone amp. This might seem odd, but the unbalanced output voltage of the DAC is 4Vrms (sic) and the input sensitivity of the headphone preamp is 300mV. I would imagine that the headphone input would be saturated with such a high voltage signal.The DAC's analogue output does have (digital) attenuation, but I prefer to keep the output as high as possible so as not to compromise the signal.

My question is this. Without using a multimeter, is it possible to estimate the voltage put out by the TVC preamp for any level of attenuation applied? The attenuator on the TVC preamp moves through 24 steps, from absolute attenuation, then 52dB of attenuation, 50dB and so on up to 0dB of attenuation.

If it is possible to estimate where the attenuator should be set on the TVC so as to allow a maximum signal voltage of 2V to be passed to the headphone amp, I imagine matters would be improved. Common sense suggests it should be set half way, but I know that these things don't necessarily work in a common sense manner!

Many thanks,

If you know what each step is or the manufacturer can tell you then it's trivial to work it out. Generally, with passive logarithmic pots, half rotation is a 20dB attenuation, or in other words, the output is 10% of the input signal. There's no way of telling what the TVC does, but I would expect it to something around that.

S

 

[wizons]

If you know what each step is or the manufacturer can tell you then it's trivial to work it out. Generally, with passive logarithmic pots, half rotation is a 20dB attenuation, or in other words, the output is 10% of the input signal. There's no way of telling what the TVC does, but I would expect it to something around that.S

Thanks for that Serge. So, if I understand you correctly, at around half rotation, which equates to about 20dB of attenuation as you say, the preamp will output about 10% of the input, or about 0.4V in this case! That's remarkable and presumably means that very little attenuation will be applied to output a signal of 2V.

As it happens I have just tried connecting the DAC directly to the headphone amp. At a subjective level, the headphones sound better connected this way rather than via the TVC preamp. There is more weight to the sound, especially in the bass. The headphone amp is valve based (my one concession to valves for no other reason than they look nice). Interestingly, the best sound of all is from my CD player's basic headphone socket, rather than via fancy DACs/headphone amps. It's a marginal difference. Perhaps that goes to show that manufacturers' refusal to fit simple headphone outputs to CD players and amps etc, is just a way of extorting more money from us.

 

[Purite Audio]

Next time I would choose a dac with variable analogue output if you intend to use it as a preamp, 4 volts might sound good in comparative dem of course!

Keith.

 

[wizons]

Next time I would choose a dac with variable analogue output if you intend to use it as a preamp, 4 volts might sound good in comparative dem of course!Keith.

Absolutely! The DAC's XLR output is in line with most balanced source equipment so there's no particular problem in matching it to most preamps, but the 4V RCA output is ridiculous and obviously designed to make punters think it's a good DAC because it's loud. I e mailed the manufacturer about this and asked if it's possible to reduce the RCA output voltage and the reply was a)such a mod wasn't possible and b) the high output voltage was necessary to match the input sensitivity of the matching power amp. A quick glance at the matching power amp's specs shows it does indeed have 4V RCA input sensitivity, which is is remarkable.

My active preamp enables input sensitivity to be varied, which is an extremely useful feature.

Why oh why can't hifi manufacturers simply standardise these matters? It would be easy to do and make hifi so much better. It seems to me that most of the effort in getting a system to sound good revolves around sorting out simple engineering matters such as this.

 

[SergeAuckland]

Absolutely! The DAC's XLR output is in line with most balanced source equipment so there's no particular problem in matching it to most preamps, but the 4V RCA output is ridiculous and obviously designed to make punters think it's a good DAC because it's loud. I e mailed the manufacturer about this and asked if it's possible to reduce the RCA output voltage and the reply was a)such a mod wasn't possible and b) the high output voltage was necessary to match the input sensitivity of the matching power amp. A quick glance at the matching power amp's specs shows it does indeed have 4V RCA input sensitivity, which is is remarkable.My active preamp enables input sensitivity to be varied, which is an extremely useful feature.

Why oh why can't hifi manufacturers simply standardise these matters? It would be easy to do and make hifi so much better. It seems to me that most of the effort in getting a system to sound good revolves around sorting out simple engineering matters such as this.

Because as long as buyers choose with their ears, the louder one will generally sound better, so there's something of a loudness war going on with output voltages slowly rising.

Pro equipment has been standardised for very many years, albeit at +4dBu nominal output for US made equipment, +8dBu for UK made equipment and +6dBu for European made equipment, although those are nominal operating levels, peak levels can vary from +18dBu up to +25dBu, so even there, it's all over the place!

The EBU has defined 0dBFS digital as +18dBu analogue, allowing for either 10dB or 12 dB headroom, but that's for uncontrolled sources, i.e. live. Controlled sources, like CD, the levels normally used are +8dBu in the UK and +6dBu in Europe both for 0dBFS.

If the professionals all have different standards, what chance for domestic users?

S.