Tisbury Audio Mini Passive Pre-amp

[Marco]

Absolutely, and worthwhile doing, as long as in the final analysis you trust what your ears are telling you, not just the measurements!

Again I have to state quite categorically that, in my set up, there is no hint whatsoever of the Tissie sounding 'etched', sharp or harsh - simply very transparent of the signal that it's being fed.

However, I could imagine that due to its brutal honesty, in some set-ups, especially those with a tendency towards forwardness [and where various 'sticking plasters' have been applied to address it], such wide-open transparency may become a bit 'too much of a good thing'.

Marco.

[Macca]

Won't a mic measurement only give you frequncy response? For evidence of harshness wouldn't you need distortion measurents?

[Marco]

I've currently got the Croft hooked up the to the Tissie, with the former acting simply as a phono stage, and as suspected playing vinyl, it sounds superb.

It may not *quite* have the airy and spacious 'out of the box' presentation of the Croft, when also being employed as a preamplifier, but it's easily good enough to listen to downstairs in the lounge on my Celestion 15XRs, whilst my normal listening room in this weather is too hot to occupy, with all the valves being used - so much so that I intend to investigate some more expensive passives, such as the Goldpoint I referred to earlier.

The Tissie has certainly been a (very welcome) eye-opener and another beneficial learning curve for me in this hobby of ours!

Marco.

[DSJR]

Something that can be predicted by measurement is possible changes doe to impedance (mis)matching. The Tis uses 10K attenuators I believe, where for whatever reason (continuity with what I know?) I use 47k or 50K, the latter in the Tis-type attenuators I'm currently using. I can't measure and have no idea what if anything I've done to the response, but it 'sounds' fine to me. Hopefully it's mostly gain that's affected...

[Marco]

How would the gain be affected, Dave? I've always wondered how the 'K' value of pots or attenuators affects the gain on the amp they're connected to

Is 10K 'higher gain' than 50K, or if not, how does it work?

Marco.

[User211]

Won't a mic measurement only give you frequncy response? For evidence of harshness wouldn't you need distortion measurents?

Well I'd only measure for FR changes, as I think distortion should be pretty negligible, but yeah odd harmonics are supposed to sound "hard". I just can't see a potentiometer generating much in that direction though. I didn't think the Tizzer sounded harsh just possibly a bit brighter accounting for the "more etched" perception. But who knows. My Alps I believe is 50K versus the 10K Tizz.

Much depends on impedance ratios and it seems to be a less than trivial topic. I'm not really into electronics in any way so I am not qualified to comment.

Of course the Tizzer should sound "tizzy". It's only logical

[cyclopse]

I've currently got the Croft hooked up the to the Tissie, with the former acting simply as a phono stage, and as suspected playing vinyl, it sounds superb.

It may not *quite* have the airy and spacious 'out of the box' presentation of the Croft, when also being employed as a preamplifier, but it's easily good enough to listen to downstairs in the lounge on my Celestion 15XRs, whilst my normal listening room in this weather is too hot to occupy with all the valves being used - so much so that I intend to investigate some more expensive passives, such as the Goldpoint I referred to earlier.

The Tissie has certainly been a (very welcome) eye-opener and another beneficial learning curve for me in this hobby of ours!

Marco.

I know exactly where your coming from regards valve heat in summer. Keep me nice and warm in winter but high summer temperatures are another issue.

Regards

Steve

[mikmas]

How would the gain be affected, Dave? I've always wondered how the 'K' value of pots or attenuators affects the gain on the amp they're connected to

Is 10K 'higher gain' than 50K, or if not, how does it work?

Marco.

I may be completely wrong on this (I usually am) but I understand the basic principle of a passive to be that of a voltage divider and therefore reductive (i.e. there is no 'gain' involved but the opposite) As the gain of the power amp is fixed it is not affected but simply supplied with a lowered voltage to work with depending on the pot position. Whether the pot is 10k or 50k will only affect the sweep of the voltage/volume curve.

I have experimented with various values with my DIY passives (using traditional Alps pots) and found 10k to be the most useful with my Quad 303 and typical 'modern' output levels.

[Marco]

As the gain of the power amp is fixed it is not affected but simply supplied with a lowered voltage to work with depending on the pot position. Whether the pot is 10k or 50k will only affect the sweep of the voltage/volume curve.

Yeah, gain is the wrong term, but you know what I mean.

By "sweep of the voltage/volume curve" do you mean how loud the sound gets in relation to the travel of the volume control? For example the sound level, with a 10k pot, at the '9 o'clock' position on a preamp, will be louder or quieter than that of a 50k pot at the same position?

If I'm talking bollocks, just say so, as I'm trying to learn

Marco.

[Firebottle]

You're talking bollocks. Yeah I've always wanted to say that

Seriously the "sweep of the voltage/volume curve" is called the law, it is usually linear or logarithmic from one end to the other. The value of the track will make no difference to the equal loudness position.
Logarithmic pots are used for volume control because the sensitivity of the ear varies with sound level, as shown by the Fletcher-Munson curves:



If you had a linear pot as the volume control all the adjustment would appear to be squashed at the top end.