DIY Passive pre-amp Advice % Help please

[serendipitydawg]

Firstly I am a rank DIY amateur. Just about able to solder phono plugs

I think I can manage a passive pre-amp but need some suggestions

I recently bought jandl100 (Jerry) Classe Seventy which sounds marvelous fed by my Tirna-modified Beresford TC 7510.

I want to be able to utilise other sources; NAD phono amp, tape decks.

Thanks in advance

[StanleyB]

How does a variable resistor amplifies the signal passively:scratch:? I am having problems with the term passive preamp. It suggests some form of perpetual motion or free energy source.

[serendipitydawg]

How does a variable resistor amplifies the signal passively:scratch:? I am having problems with the term passive preamp. It suggests some form of perpetual motion or free energy source.

My terminolgy may be a bit awry Stan. It would be wonderful if your products had a single line input

Both my TC7510's are cherished possesions

[John]

I have a couple of suggestions
http://www.audiotweaks.com/diy/bentaudio/page01.htm This will be on the same playing field as a Music First pre amp as uses the same transformers
http://www.hificollective.co.uk/kits/passive_description.html nice and cheap should sound good
http://www.tnt-audio.com/ampli/django_e.html
A friend uses this not cheap but performs really well
Some people say passive can be bass shy I think it depnds how you implement them myself

[Ali Tait]

More a case of impedance varying with volume than anything else IMO John.It's a good idea to use a buffer when using a passive pre.

[John]

Yes I like what the a buffer does too a passive

[Steve Eddy]

How does a variable resistor amplifies the signal passively:scratch:? I am having problems with the term passive preamp. It suggests some form of perpetual motion or free energy source.

Yeah, it's an oxymoron alright. At least for resistive units. But that's unfortunately the term that's "stuck" ever since they started gaining popularity back in the 80's.

However you can achieve signal gain passively through the use of transformers. And in fact some of the transformer-based volume controls out there can offer 6dB of gain, making them truly "passive preamps."

se

[Ali Tait]

And very good they can sound too,in the right set-up.

[Steve Eddy]

And very good they can sound too,in the right set-up.

Yes.

Though personally I feel that what primarily accounts for that is the transformer itself rather than its function as an attenuator. Before TVC's came onto the scene, about the only use of transformers in consumer audio was in tube gear and even then you're mainly only talking about output transformers and in some cases interstage transformers. No one to speak of had ever used good quality input transformers.

I'd been using them for years and wouldn't build anything without them.

Of course TVC's and AVC's can help with regard to output impedance compared to resistive attenuators and don't have the issues with cable capacitance and high frequency rolloff. Though personally I prefer a fixed input transformer coupled to a resistive attenuator with the whole thing built into the amplifier chassis. But if you must have the attenuator separated from the amp, TVC's and AVC's can work rather well.

se

[serendipitydawg]

Thanks to all who have responded.

Thanks, in particular, to John who has actually tried to answer my question rather than go off into a discussion of the merits of transformers, which is neither relevant to me or my original question.

I had in mind something like this

http://www.hi-fi-insight.com/custom-passive-pre-amp.html

Which in fact led me to hificollective.co.uk, in search of a stepped attenuator which hifi insight seemed to suggest was available for £45+ VAT (more my sort of price range)

I found hificollective.co.uk complicated and "expensive".

So perhaps what I am after isa volume control in a box perhaps for £100 all in. Switching of sources is optional. I will reconfigure my system to listen to different sources if necessary.

Can't find a Creek OBH12 or QED MA34 available secondhand, either of which would be ideal

[lurcher]

Just for info. The current HFC equivalent of the vishay switch in that article is the Takman resistors on 24 way


stereo on 24way gold - £40.00+vat+p&p(kit) - £60.00+vat+p&p (built)

http://www.hificollective.co.uk/catalog/glasshouse-takman-stepped-attenuator-p-4601.html

You can find the switch in the article on ebay

http://cgi.ebay.co.uk/Dale-23-Step-Attenuator-Volume-Control-Log100k-Stereo-/320573784217?pt=LH_DefaultDomain_0&hash=item4aa3afc099

[Techno Commander]

I saw this item recently and it appears to be very well made for the price.

Ebay (http://cgi.ebay.co.uk/HiEnd-attenuator-48-steps-silver-passive-preamp-Caddock-/120617580753?pt=LH_DefaultDomain_0&hash=item1c155e38d1)

[serendipitydawg]

Thanks Nick. That eBay item looks exactly like what I need.

Any idea what value I would need?

The Classe manual states

Sensitivity: 900mV for rated output Input Impedance 70 Kohms.

Can you please advise?

Cheers

[lurcher]

Its more a matter of what you are driving it from. with the amp having a input imp of 70k, I would be temped to suggest 50k, or 25k even if the thing you are driving from is a low impedance source. That is the big problem with resistive passives the impedance matching is always worst than if they were not in the chain. Also get it as close to the power amp as possible. A passive can't drive any length of line.

[Steve Eddy]

Thanks Nick. That eBay item looks exactly like what I need.

Any idea what value I would need?

The Classe manual states

Sensitivity: 900mV for rated output Input Impedance 70 Kohms.

Can you please advise?

I'd recommend a 10k unit. And avoid sources with output impedances greater than 1k. That rules out some of the more asthmatic tube output stages, but leaves the field pretty open.

se

[Reid Malenfant]

Before you go jumping in at the deep end i think i ought to mention this seeing as you mentioned being a rank amateur ;)

Please remember that unless your source can & does output more voltage than your power amp needs to fully drive it you'll never be able to turn the volume up fully. This is thing with passives, you'd be ok with a CD player as these can output up to 2V RMS & most power amps probably require 700 - 800mV to drive them fully. Again though there are exceptions, my KSA250 needs over 2V RMS to fully drive it :eyebrows:

Most phono pre amps output only 200 - 500mV so don't expect your amp to go as loud as normal, you won't be able to fully drive it :doh:

[serendipitydawg]

Nick

I am trying to sort out a means to listen to analogue sources into my new power amp whilst I seek out a suitable bargain preamp.

I would forgo source switching which would mean mean less soldering ( which actively irritates me) but obviously need a volume control. Most basic setup I envisage would be stepped attenuator with one flying phono lead for input and one flying phono lead for output. Both as short as possible.

Sources would be casstte deck line out, reel to reel line out and most importantly NAD PP2 phono preamp line out

I am currently using my Beresford TC7510 as a preamp fed by 4 digital sources and it sounds wonderful

[Reid Malenfant]

Sources would be casstte deck line out, reel to reel line out and most importantly NAD PP2 phono preamp line out
Hi Matthew, all those sources will not output enough to turn your power amp up so it puts out it's stated power.

They'll all only put out about 200 - 500mV if you are lucky, i'd suggest nearer 200mV :( If you happen to have a 30W amplifier then expect it (with 800mV input) to put out a maximum of 2Watts per channel.

Trust me i'm not joking :rolleyes:

[serendipitydawg]

Hi Mark

It's Matthew aka serendipitydawg

I said right at the beginning that I was a "rank amateur" so here's a real rank-amateur question.

"If I plugged the NAD PP2 phono stage directly into the power amp it wouldn't blow tweeters or eardrums. Right?"

I can live without volume control until I can afford a preamp

Cheers

[Reid Malenfant]

No it won't, that's just what i'm attempting to explain :) Not all things are created equal & in the industrys' wisdom no-one really settled on a standard sensitivity except on more pro/studio based gear.

Tbh i don't know what power amp you are using so perhaps i should have said "it's not likely to blow anything up" rather than "no it won't" :lol:

Most, though not all power amps have input sensitivies at approximately 800mV, as i pointed out earlier my KSA250 needs 2.5V or more to drive it to full power. Some are more sensitive & yes a few hundred mV (200mV) might well get them to full power. Chances are though that you'll need 800mV to get full power out of your amp.

As mentioned previously nearly all analogue front end stuff is designed to put out about 200mV at full output, this is why we have a pre amp to boost the signal & full drive the power amp.

CD & DAC outputs are different as they are all designed to stick out 2V RMS (10 times the voltage of analogue stuff) at full output. If you were just using a CD player then yes, you'd be fine with a passive pre amp as there should be more than enough voltage to fully drive the power amp (Krell & others would still need some gain to be driven fully).

I hope that helps, if not fire away with the questions & i'll do my best to help out ;)

[Ali Tait]

Fair point,but when do you ever turn your volume pot all the way up?

[Reid Malenfant]

Fair point,but when do you ever turn your volume pot all the way up?
I never do, but that's because i'd start overloading the power amps as i'm using a pre amp ;)

Take it from me, the scenario i painted earlier is bang on. With 200mV output from an analogue device on the front end & a power amp with 800mV sensitivity for full output, the most the amp could put out would be 1/16th the full power :doh:

[serendipitydawg]

Hi Mark

I've connected the NAD PP2 directly to my Classe Audio Seventy power amp. Your initial judgment was correct; no blown tweeters or eardrums! But it was still quite loud

So if I've been following you correctly the 10K stepped attenuator is the one to go for? I just want to be able to reduce volume slightly. It's only slightly less than £30 including delivery.

Isn't the spirit of DIY "give it a go"?

[Reid Malenfant]

Just remember that you'll never be able to turn up the volume any higher than it was when you just tested it a little while ago ;) As long as you are happy with that then yes, like Steve Eddy said i'd suggest the 10K switched attenuator.

Any higher & you'll actually have a more logarithmic volume control as the power amp will load the whole circuit on top of the control itself. Most if not damn near all front end stuff shouldn't have any problems at all with diving a 10Kohm load in parallel with something like 47Kohm (power amp input impedance).

If you desired in the future you could add a simple gain stage that only connects when using an analogue front end. You could fit a CD monitor switch (double pole double throw) & bypass the gain stage when listening to CD. All other analogue inputs could be fed through the gain stage.

A simple battery operated circuit using something like an NE5532 dual op amp would be more than sufficient :)

[serendipitydawg]

Thanks Mark!

I'll give it a go and report back

Cheers:cool:

[lurcher]

Why not get a simple dual pot to try it out. If it works well enough, then replace the pot with a stepped attenuator.

http://www.maplin.co.uk/Module.aspx?ModuleNo=2204&OrderCode=JM83E

[serendipitydawg]

Good idea

Shows how much time I spend on AOS and similar, I just had it in my head that potentiometers are "bad" from reading about the experiences of others

[lurcher]

They are far from ideal, but its worth it to find out if its worth taking further. If you have spare gain, set them up as a shunt with a good resistor. but just try the simple setup first and see how the levels work out.

[Ali Tait]

Yes,shunting a cheap pot with good resistors will give most of the sound of a pricey stepper.

[Rare Bird]

I find Linear Pot with Law Faking Resistor more acceptable pot to use..Linear are more cleaner than log..

[Steve Eddy]

The whole "shunt attenuator" thing is based on a myth. Namely the myth that the "signal path" only constitutes the series elements in a circuit.

The idea is that you use a single, high quality resistor for the series element, and then you can use a lesser quality resistor (or pot as the case may be) for the shunt element as it's not "in the signal path."

Nothing could be further from the truth and this idea was dreamed up by those who don't have any understanding of how things work (which is how a lot of "audiophile" mythology gets started).

In terms of quality, the shut element is every bit as critical as the series element as the "signal" is the voltage dropped across the shunt element as a consequence of the current flowing through it.

se

[Steve Eddy]

I find Linear Pot with Law Faking Resistor more acceptable pot to use..Linear are more cleaner than log..

What it is is that linear pots have better element-to-element tracking than log pots.

The drawback can be their presenting the source with a varying load depending on position, but that's usually not a problem in most instances.

se

[Haselsh1]

Someone mentioned the Creek OBH passive preamp a while back. I really cannot recommend this unit as from personal experience, it deadens the sound and sucks the life out of the music's dynamics. I used one with two Audiolab 8000M amplifiers and it wasn't until I substituted a Croft Micro 25 Basic that I really found out what the power amps were capable of.

[Reid Malenfant]

Why not get a simple dual pot to try it out. If it works well enough, then replace the pot with a stepped attenuator.

http://www.maplin.co.uk/Module.aspx?ModuleNo=2204&OrderCode=JM83E
Apologies as i really don't mean to be picky, but i thought the whole idea behind this was to 1. Do away with the active circuitry of a pre amp & 2. Use a decent quality volume control to see what the difference makes ;)

I honestly wouldn't conceive of using a cheap & nasty thing like that Maplin potentiometer when the volume control pot in Matthews present pre amp or whatever has a good chance of being something reasonable like an Alps pot.

Imo you either use a decent pot or stepped attenuator or you just simply don't bother. Cheap ingredients can often leave a nasty taste in the mouth :eyebrows:

At least get the one thing right as it'll all be depending on the 'one thing' :) Imo

[Ali Tait]

The idea of trying the cheap pot was to see if the 10k value worked ok.Daft spending a lot of money on a nice stepper only to find the value doesn't suit!

[Reid Malenfant]

True, though as i already think i stated all the front end kit should have zero problems driving a 10K & 47K in parallel. Unless you happen to be John Curl & stuff a 1K resistor in series on the output of something that is :doh:

Output impedance of most stuff is pretty low & i'd be surprised if it was more than 250 ohms on virtually any solid state kit that has been made in the last 40 years. Most should be well under 100 ohms :)

Driving it should be no problem at all, though as you say i can't hurt to try out a cheapy (except your ears) :lol:

[Steve Eddy]

True, though as i already think i stated all the front end kit should have zero problems driving a 10K & 47K in parallel. Unless you happen to be John Curl & stuff a 1K resistor in series on the output of something that is :doh:

What'd he do that on?

se

[Ali Tait]

The whole "shunt attenuator" thing is based on a myth. Namely the myth that the "signal path" only constitutes the series elements in a circuit.

The idea is that you use a single, high quality resistor for the series element, and then you can use a lesser quality resistor (or pot as the case may be) for the shunt element as it's not "in the signal path."

Nothing could be further from the truth and this idea was dreamed up by those who don't have any understanding of how things work (which is how a lot of "audiophile" mythology gets started).

In terms of quality, the shut element is every bit as critical as the series element as the "signal" is the voltage dropped across the shunt element as a consequence of the current flowing through it.

se

Myth or not,co-incidentally I'd shunted the Alps Blue in my Pre 2 a couple of weeks ago with some Audio Note tantalums,and the improvement was not subtle.It has improved even more over the last week as they have burned in.I used the 2w versions.

[lurcher]

I fully agree the idea of signal path not passing through the pot is confused and wrong as far as network analysis is concerned. So I agree with Steve there. I never said when I suggested the use of a shunt, that the myth he mentions is involved. But as Ali has said observed results seems to indicate that for whatever the reason the effect is beneficial.

[Ali Tait]

Indeed Nick,in fact I'm still noting improvements now.Wonder if it's because I'm using 2w when a quarter watt will do,so it takes longer to burn in.Whatever the reason,this is well worth doing for the cost of two resistors. To give an idea,the improvement is as great if not greater then changing a coupling cap in a valve amp/preamp.

[Reid Malenfant]

What'd he do that on?

se
I can't remember, however it was customer specified if i remember correctly ;)

[Steve Eddy]

Myth or not,co-incidentally I'd shunted the Alps Blue in my Pre 2 a couple of weeks ago with some Audio Note tantalums,and the improvement was not subtle.It has improved even more over the last week as they have burned in.I used the 2w versions.

Great. Glad you're enjoying it.

At the end of the day we like what we like whatever the reasons may be.

se

[Steve Eddy]

I fully agree the idea of signal path not passing through the pot is confused and wrong as far as network analysis is concerned. So I agree with Steve there. I never said when I suggested the use of a shunt, that the myth he mentions is involved. But as Ali has said observed results seems to indicate that for whatever the reason the effect is beneficial.

Well, for most anything you care to name, there will be those who love it and those who hate it. So "beneficial" is often quite relative. There are few absolutes out there.

se

[Steve Eddy]

I can't remember, however it was customer specified if i remember correctly ;)

Ah. The customer wasn't Parasound was it? :lol:

se

[serendipitydawg]

My first foray into DIY will have to wait till the weekend when I can get to the local branch of Maplin and buy the potentiometer recommended by Nick.

Let me reinforce my rank amateur status by saying that just looking at the picture on the Maplin website makes me wonder; which tags are input which are output, and if there's three tags presumably thats left right & common earth?

I really appreciate all you knolwledgable types taking me seriously; so any hints or tips will be much appreciated.

Cheers

[Steve Eddy]

My first foray into DIY will have to wait till the weekend when I can get to the local branch of Maplin and buy the potentiometer recommended by Nick.

Let me reinforce my rank amateur status by saying that just looking at the picture on the Maplin website makes me wonder; which tags are input which are output, and if there's three tags presumably thats left right & common earth?

You were probably looking at a photo of a single potentiometer. For two channels, you'll need two. Either two separate potentiometers, or what's called a "dual gang" which is two potentiometers that share a single shaft.

Each potentiometer, whether separate or dual gang will have three tags on it. With the shaft pointing toward you and the tags pointing upward, the tags will be, from left to right, in, out and ground.

http://www.q-audio.com/images/pot.jpg

And here's an example of a dual gang pot:

http://www.q-audio.com/images/pot2.jpg

se