PDA

View Full Version : Active Preamplifiers - Improving a weak link?


Brian
04-02-11, 09:56 AM
I think this is a bit off the wall but I'll post it anyway...

I've been following the preamplifier threads with some interest, I don't fit in the camp of believing an active preamp will always improve any system as some people at least appear to believe but neither do I reject them out of hand having owned plenty in the past.

In my valve integrated amp I've changed volume control from
Alps Blue > TKD > DACT Stepped Attenuator > Passive TVC
and at each stage there has been an improvement.

If I owned an active preamp right now and it was supplied with something like an Alps Blue pot I would immediately swap it out for the DACT for an overall system improvement. Does it follow that a further improvement could be had by completely removing the volume pot from an active preamp, connecting the preamp output to a passive TVC?

Possibly a daft idea but I'd like to open it up for discussion.

martin clark
04-02-11, 11:48 AM
No, it doesn't: TVCs are quite sensitive to loading, and rather, demonstrably, less linear than even a basic attenuator /pot.

Get it right and it could be very good. But if you want active, then you can pay just as much attention to implementing a simple attenuator well. That may not be perceived as such if your preferred approach is 'flavour to taste' (Nothing wrong in that BTW, becasue IMO an audio system is about personal pleasure. Let's just ignore all claims about 'accuracy')

But for the effort involved, I'd keep it simple - take a good attenuator and follow with a buffer, apply bandwidth -limitation appropriate to context and call it done. And that's how my preamp works ;)

Robert
04-02-11, 02:01 PM
+ tone controls, with a direct option of course.

Brian
04-02-11, 02:07 PM
+ tone controls, with a direct option of course.
Tone controls? Am I missing a joke here, Robert?

ChristianThomas
05-02-11, 12:01 AM
+ tone controls, with a direct option of course.

Quite. Why not. Just because some tone controls are done badly doesn't mean they all have to be. We've all gone rather snotty in thinking tone controls are a bit infra dig.

Maybe the problem is with the Baxandall tone control. And for the purist who thinks anything above Zen is too much, another two capacitors in the signal web is going to be pretty hard to detect in a finished design. Besides, what were the designer's listening preferences beside being personal tone controls.

Bring them back I say. I've got plenty of recordings where the balance could do with some Eq. I've got plenty where one could even usefully start again.

CT

Brian
05-02-11, 02:58 AM
Quite. Why not. Just because some tone controls are done badly doesn't mean they all have to be. We've all gone rather snotty in thinking tone controls are a bit infra dig.

Maybe the problem is with the Baxandall tone control. And for the purist who thinks anything above Zen is too much, another two capacitors in the signal web is going to be pretty hard to detect in a finished design. Besides, what were the designer's listening preferences beside being personal tone controls.

Bring them back I say. I've got plenty of recordings where the balance could do with some Eq. I've got plenty where one could even usefully start again.

CT
It's nothing to do with being snotty. At least not for me. My son has tone controls on his amp but never uses them. Even before I discovered amps without tone controls and had a Technics amp with them, I never felt I needed to use them so I just don't see the point.

DevillEars
05-02-11, 03:26 AM
I think we're focusing too much on the usage of tone controls to apply correction to a signal between source and speakers...

The one element over which we have limited physical control (apart from selection and some either minor aesthetic or major plain damn ugly countermeasures) lies in the listening room's acoustics.

In my youth (way back then), the listening room was also the only room (a bachelor flat) and suffered from serious room mode challenges. The main hassle was resonance in the lower frequency ranges, and I remember minimising this problem by applying some bass cut on the then incumbent B&O Beolab 5000 integrated. I seldom, if ever, used the tone controls for anything else.

Room problems still persist and a friend made very successful use of a Z-Systems RDP-1 (digital domain parametric equaliser) to counter some serious room vagaries in an environment that consisted of an area of some 8m x 4m with floor-to-ceiling windows for two sides and large archways into adjoining rooms on the remaining two sides.

Listening to his system (Meridian CDP and Meridian Active "Digital" speakers) with and without the correction applied was quite an eye-opener (or should that be "ear-opener"?).

Without the correction, it was almost painful to listen with numerous resonance points and hellish reflections from the glass (rear and to the right), while the archways sucked life out of the presentation.

With the correction applied, everything fell into place and the sound heard in the listening position was what one would expect from such a system.

For vinyl users, the RDP-1 may not be that attractive as it introduces the need for an ADC between phono stage and correction as the RDP-1 applies its magic only in the digital domain, but - in his system - it made an otherwise unlistenable system sound the way its designer intended.

So, rather than knee-jerk and discount tone control circuits out of hand, one should consider the potential for them to help correct some aspects external to the electronics and thereby improve the listening experience.

The alternative is spending a fortune on a custom-designed listening room - something not all of us can afford.

:cool:

Steven Toy
05-02-11, 04:58 AM
The objectivists crap on the thread with this guff about tone controls. Extra capacitor and switches in line will compromise transparency.

The grounded grid pre I use has been modified a couple of times since I've had it by the guy who originally built it. The ALPS Blue > DACT stepped attenuator plus silver cable brought more depth, better dynamics.

More recently the PSUs were moved into a separate box and the signal path shortened. The result is more realistic instrumental tone and texture. Why would I want to lose some of that for poxy tone controls that I'm never going to use?

sean99
05-02-11, 05:13 AM
Extra capacitor and switches in line will compromise transparency.

How many components are in the signal chain in a typical mixing desk ?
What about in the equalization during mastering ?

Steven Toy
05-02-11, 05:48 AM
Why pile on the misery (or add further equalisation) during playback?

Your logic is it's crap during recording so making it even worse won't make any difference. The audio chain is about minimising signal losses. It's never to late to save what's left.

Robert
05-02-11, 06:36 AM
The objectivists crap on the thread with this guff about tone controls. Extra capacitor and switches in line will compromise transparency.



LOL - tone controls adjust sound balance to the preference of the listener.

You won't find anything more subjective!

Robert
05-02-11, 06:53 AM
Why pile on the misery (or add further equalisation) during playback?

Your logic is it's crap during recording so making it even worse won't make any difference. The audio chain is about minimising signal losses. It's never to late to save what's left.

All this talk of minimising losses is just that - talk.

I'll gladly demo a set of signal caps and switches in line and neither you nor anyone else will detect them. If anyone really does want to try this please PM and I'll be delighted to replace the myth with some facts via demo.

On applying tonal correction I would say two things:

A loudspeaker will show a different tonal balance across a range of rooms. You have three ways to adjust this - room positioning, acoustic treatments and electronic correction. The first is the most important to get right but doesn't always solve a problem, so we should then consider the remaining two solutions.

You are extremely unlikely to be using the studio monitor loudspeaker used to master your music and you certainly wont have a listening room that replicates the studio, or the ears of the engineer.
It is therefore highly probably that what you hear at home is very different to what was heard by the engineer.
You might like it and that's great - nothing more need be said.
You might not, and in this situation having the ability to partly correct tonal balance could bring you greater enjoyment. It clearly does to the vast majority of people who do use tone controls (a few hair-shirters on a forum really isn't typical).

Steven Toy
05-02-11, 07:00 AM
Objectivists say tone controls make no difference to transparency and integrity the signal. There's a surprise!

The option of setting the tonal balance is one I will happily trade for texture and immediacy. I don't subscribe to the sticking plaster approach. This is mediocre-fi.

Brian
05-02-11, 07:01 AM
ffs hairshirters? Personally I haven't found any advantage to having tone controls even when I've had them. I couldn't give a shite whether other people like them or not, though, each to their own and all that.

I guess I must have missed something along the way by not realising this was such a contentious issue. I wish I hadn't started the damn thread now, I'll know better next time not to ask any questions.

Robert
05-02-11, 07:06 AM
Objectivists say tone controls make no difference to transparency and integrity the signal. There's a surprise!

The option of setting the tonal balance is one I will happily trade for texture and immediacy. I don't subscribe to the sticking plaster approach. This is mediocre-fi.

We don't say it Steven, we prove it.

Robert
05-02-11, 07:13 AM
ffs hairshirters? Personally I haven't found any advantage to having tone controls even when I've had them. I couldn't give a shite whether other people like them or not, though, each to their own and all that.

I guess I must have missed something along the way by not realising this was such a contentious issue. I wish I hadn't started the damn thread now, I'll know better next time not to ask any questions.

Oh come now Brian, you are hardly afraid of contentious issues. I do venture into other rooms now and then ;)

Hair-shirt = minimalist.
People like Steven who adopt a rigid dogma that says a signal path must be (visually) ultra simple.

But yes, passive v active and straight line v tone have always been contentious issues. Well from the 1970's at least.

Jackytwoshoes
05-02-11, 07:19 AM
I work as a sound engineer but I'm also keen on hi fi.
At a gig for instance, you'd have the setup parametrically EQ'd with some surprising cuts at problem frequencies. At home, I baulk at an extra centimeter of cable.

They're two different things in my head:

"Hi fi is to sound engineering as cosmology* is to astronomy".

* when being particularly cutting, replace with astrology.

Steven Toy
05-02-11, 07:19 AM
Brian, the thread was crapped on by the mention of tone controls. I have no need for them any more than you. Back to OP I've tried all the permutations you mention. DACT stepped attenuator is the daddy. Passive preamps sound dynamically flat unless the runs of cable connecting them to the power amp are very short which is why they work best in integrated amps.

Brian
05-02-11, 07:20 AM
Oh come now Brian, you are hardly afraid of contentious issues. I do venture into other rooms now and then ;)

Hair-shirt = minimalist.
People like Steven who adopt a rigid dogma that says a signal path must be (visually) ultra simple.

But yes, passive v active and straight line v tone have always been contentious issues. Well form the 1970's at least.
I asked a serious question in the opening post so thanks for crapping on the thread. Tone controls have exactly nothing to do with what I was originally asking.

Not only couldn't I give a shite whether people want tone controls or not I don't give a shite what you and Steven think about tone controls. I suggest you start a new thread about tone controls or better still get into a PM exchange with him about it.

Edit: In case you don't quite get it, by contentious issue I'm talking about my opening question, not your thread crap about tone controls.

Brian
05-02-11, 07:25 AM
Brian, the thread was crapped on by the mention of tone controls. I have no need for them any more than you. Back to OP I've tried all the permutations you mention. DACT stepped attenuator is the daddy. Passive preamps sound dynamically flat unless the runs of cable connecting them to the power amp are very short which is why they work best in integrated amps.
Steven

Interesting comment there. I have a DACT stepped attenuator sitting in a box having replaced it with a Django TVC. The improvement wasn't as big from DACT > TVC as the other improvements through volume pots but the TVC is superior in my system.

Steven Toy
05-02-11, 07:53 AM
I tried a Glasshouse TVC and I base my opinion on what it did in my system. I liked its transparency buy not its restrained dynamics and occasional restlessness. The grounded grid I use gives me added life, balls and a pleasing 3D aspect, especially since the DACT attenuator replaced the ALPS Blue pot.

S-Man
05-02-11, 08:32 AM
.... Does it follow that a further improvement could be had by completely removing the volume pot from an active preamp, connecting the preamp output to a passive TVC?

Possibly a daft idea but I'd like to open it up for discussion.

It's all system dependent!!!

One likely benefit of a TVC is that it breaks ground loops and another is that it limits the bandwidth. My view is that in a correctly designed and implemented system an Alps Blue will give better audible and measured performance than a TVC and at much lower cost.

Brian
05-02-11, 09:29 AM
It's all system dependent!!!

One likely benefit of a TVC is that it breaks ground loops and another is that it limits the bandwidth. My view is that in a correctly designed and implemented system an Alps Blue will give better audible and measured performance than a TVC and at much lower cost.
Thanks.

My Ella valve amplifier from diy hifi supply was supplied with an Alps Blue, I can't see what I might change in the implementation that would result in it improving on the other options I've lived with...the TKD, the DACT and the TVC.

I'm only interested in the audible performance by the way. I wouldn't switch back to the Alps Blue (I still have that too) no matter how much better it measures than the other options.

Edit: If anyone has a diy hifi supply "Basie" they don't want I may be interested in trying all this out if the price is reasonable. :)

Robert
05-02-11, 11:58 AM
I'm only interested in the audible performance by the way. I wouldn't switch back to the Alps Blue (I still have that too) no matter how much better it measures than the other options.



If you are going to compare passive solutions you need to know both the drive and load conditions. Both determine what works well and what flops.

So an Alps Blue could easily work superbly in one circuit but be a disaster in another. Plus there are many versions, so a 50k Blue might work well and a 20k less well.

Brian
05-02-11, 02:44 PM
If you are going to compare passive solutions you need to know both the drive and load conditions. Both determine what works well and what flops.

So an Alps Blue could easily work superbly in one circuit but be a disaster in another. Plus there are many versions, so a 50k Blue might work well and a 20k less well.
Yes, I'm aware of that, however I'm not specifically comparing passive solutions. I'm talking about whether there may be any benefit to adding an active preamplifier to my system, removing the volume pot and using it along with a passive TVC.

Robert
05-02-11, 07:24 PM
A TVC with an active unity gain buffer at each end?

No benefit at all.
Transformers add distortion, pots don't.

Martin gave the definitive answer in post 2.

James
05-02-11, 07:55 PM
I've just had a dastardly clever idea. A very useful 'tone control' would be one that compensates for baffle step. It would both cut or boost the LF with selectable F3 frequencies, hence accommodating a wide range of loudspeakers in a wide range of room configurations. Imagine, SBLs and Kans no longer need to be wall mounted and meaty american behemoths can be installed in puny British rooms with impunity.

Oh wait, isn't that how the parametric EQ works in Quad preamps?

Brian
06-02-11, 03:18 AM
A TVC with an active unity gain buffer at each end?

No benefit at all.
Transformers add distortion, pots don't.

Martin gave the definitive answer in post 2.
Right, so let's go backwards and keep in mind I'm not remotely interested in how things measure, just how they sound.

If I remove the Django TVC and replace it with (for example) a Basie active preamplifier complete with Alps Blue volume pot my system will sound like 'x'.

If I swap the Alps Blue in the Basie for a TKD volume pot and then for a DACT stepped attenuator it will sound different with each one, let's say it will sound like 'y' and 'z'. Whichever of 'x', 'y' and 'z' sounds the best is down to personal preference but they will all work.

Are you saying that removing all of these volume controls and connecting the output of the Basie directly to the TVC is fundamentally a bad thing that can't sound good, even though the TVC currently sounds better than them all in my current setup?

Thanks in advance.

mudlark
06-02-11, 04:37 AM
Brian,
I have a creek obh-22 which is an alps blue based remote control passive. i added a B4 amplifier as a buffer and it sounded better to my ears. i also did the same with a naim 32.5. (This worked OK actually and gave a reasonable result......) the naim 32.5 wasn't as good a buffer as the B4.

I therefore completed my b4 project adding a remote control alps blue and input control to make an excellent preamplifier. The b4 is a buffer and adds no gain.

I am going to try a relay based volume control to compare things with the alps b4. This, if working well will become my main preamp or a headphone amp.