Are passive preamps bad?

[Rainier939]

Hello,

In a discussion on one of my previous post the topic of pasive preamp stages came up. This person claimed that they were not ideal and should be avoided.

I didn't follow up why that was because I didn't want to go to much off topic.
But I want to ask the question now.

Why or if they should be avoided?

In my mind they make sense.
The dac plays it's full range signal that I would believe is it's optimal ouput level.
There is no processing needed in your dac/streamer.
No circuits to that can add distortion and/or noise.
You lower the noise comming in from the input device.

It's an input selector with a potentiometer in a box. What could go wrong

The only reason I can think of to avoide them is the non linearity of the potentiometer channels.

Am I missing something here?

 

[Blumlein 88]

You need to use them in situations that mitigate possible issues. They have a high output impedance. So using them with short low capacitance cabling is needed or you'll get a high frequency roll off. They also need to be used with sources that don't need a higher input impedance. You can usually manage that with 10 k ohm pots. Also as they provide no gain make sure the output is enough for your needs.

Some use switched resistors instead of pots. That eliminates channel to channel balance issues at lower volumes.

So be careful of those pitfalls and they can be just wonderful. OTOH, we have very inexpensive totally transparent active preamps available.

 

[Roland68]

You need to use them in situations that mitigate possible issues. They have a high output impedance. So using them with short low capacitance cabling is needed or you'll get a high frequency roll off. They also need to be used with sources that don't need a higher input impedance. You can usually manage that with 10 k ohm pots. Also as they provide no gain make sure the output is enough for your needs.

Some use switched resistors instead of pots. That eliminates channel to channel balance issues at lower volumes.

So be careful of those pitfalls and they can be just wonderful. OTOH, we have very inexpensive totally transparent active preamps available.

Can you please name a few?

 

[NTK]

Can you please name a few?

Any headphone amp can double duty as a preamp. Quite a few provide the appropriate connectors to make them function as one. Search the review index and you will find plenty of good options.

 

[Blumlein 88]

Passives? I made most of mine. Some checked out here on ASR.

Review and Measurements of Schiit Saga Tube Pre-amp

This is a review and detailed measurements of the Schiit Saga passive/tube pre-amplifier. It is on kind loan from a member who has waiting literally months for his review! The Saga costs USD $349 from Schiit direct with stock tube plus shipping. The tube supplied with this unit was the...

www.audiosciencereview.com

Has a tube and a passive option. Both are good.

Review and Measurements of Schiit Sys Passive Pre-amp/Switcher

This is a measurement and review of the Schiit Sys switcher and passive pre-amp. It retails for just $49 plus shipping which is reasonable. The unit is on loan from a member so not new. I will do a teardown later but for now, there are just two inputs, one output, and a volume control in...

www.audiosciencereview.com

Passive, simple switcher and volume control. $49.

Active pres, JDS makes one, some of the Schiit gear, there are others I'm not thinking of at the moment.

 

[Blumlein 88]

Any headphone amp can double duty as a preamp. Quite a few provide the appropriate connectors to make them function as one. Search the review index and you will find plenty of good options.

I started to post something similar. Lots more headphone amps have been reviewed here. So more options to choose from.

 

[solderdude]

Am I missing something here?

They don't require power.
They pass DC too (which could be an issue).
Are not limited in frequency range, other than by the capacitive load. The latter is one of its downsides (varying input and output resistance that is volume setting dependent.
Potmeters/stepped attenuators without a transformer being used can only attenuate.
When used with vinyl needs an external pre-amp.
Can not be used with older gear as they usually have a too low output voltage.

 

[sergeauckland]

Even capacitative load isn't too serious a problem, as using some low capacitance cables like RG6 or RG59 runs up to 5-6 metres don't attenuate HF in the audible range. Passing DC needn't be a problem either, a single capacitor per channel sorts that one out.

I used 5m of RG59 with a 10k passive pot for years with my Meridian M2 Active 'speakers without any issues. The only issue can be lack of gain with some sources, like FM tuners, but for digital sources which normally output at least 2v, that's enough for most power amps or active 'speakers.

S

 

[Ken Tajalli]

I believe there was/is a company that makes transformer based volume controls with relay based operation and a Remote!
The transformer solves impedance issues, DC issues, but alters the max. output level a bit.

 

[solderdude]

The transformer solves impedance issues, DC issues, but alters the max. output level a bit.

It makes impedance issues even worse and limits the frequency response on both ends of the spectrum, adds distortion and has a limited input voltage for the lower frequencies and isn't as 'flat' in impedance as a regular attenuator.
I would take a regular passive or active pre-amp over such a device (as well as over LDR volume controls) when gain is not needed.
What it can solve (at a cost) is the lack of gain in attenuators.

Fortunately the human hearing isn't as fussed about this as measurement equipment is and a substantial amount of 'modification' of the original signal can occur before it becomes objectionable. Placebo also can do a lot for audio consumers as well.

I use an attenuator + some switches too as pre-amp... it is just followed by an op-amp ensuring a low and not varying output resistance and is adding a little gain for when I need it.

 

[JiiPee]

As others have mentioned, passive pre-amps have some limitations, but in most cases they should be ok as long as:
- You use fairly short interconnections.
- Your source devices provide strong enough output signals considering the sensitivity/gain of the power amp.

Then again, high quality active pre-amplification (at least from sound quality perspective) is available at relatively low prices nowadays either as dedicated pre-amps, or as headphone amps with extra pre-amp out connections. For example the offerings from Schiit, Michael Fidler, JDS Labs etc... or the numerous Chinese manufacturers, if You are willing to go that route...

 

[Ken Tajalli]

It makes impedance issues even worse and limits the frequency response on both ends of the spectrum, adds distortion and has a limited input voltage for the lower frequencies and isn't as 'flat' in impedance as a regular attenuator.
I would take a regular passive or active pre-amp over such a device (as well as over LDR volume controls) when gain is not needed.
What it can solve (at a cost) is the lack of gain in attenuators.

Fortunately the human hearing isn't as fussed about this as measurement equipment is and a substantial amount of 'modification' of the original signal can occur before it becomes objectionable. Placebo also can do a lot for audio consumers as well.

I use an attenuator + some switches too as pre-amp... it is just followed by an op-amp ensuring a low and not varying output resistance and is adding a little gain for when I need it.

You missed the point!
- it is complex
- It costs a lot to make
- It would be expensive
- It avoids active evil circuitry in the signal path.
All good selling points.
Who needs a preamp these days? just a switch box will do for most.

 

[Waxx]

Passive preamps have advantagess, but mainly issues i think. I had a relative good one (Tishbury passive preamp) and the output impendance presented to the amp varies with the volume. At the end i did put a circuitboard with an active buffer (with an opamp) behind it to conquer that. And that does what the passive preamp tries to do, a no gain no colouration preamp device. This is a very simple and small circuit, that can be run from a 9V battery even (if you don't want a powercord) as it draws little current. I've used that for a while and it sounded a lot better than the tishbury with the opamp buffer. The opamp used was the very generic NE5532 that costed a few cents, in a very basic buffer circuit fed from a small SMPS. The total circuit was so small it fitted the original tisbury box.

 

[Roland68]

Any headphone amp can double duty as a preamp. Quite a few provide the appropriate connectors to make them function as one. Search the review index and you will find plenty of good options.

That wasn't my question, I was hoping @Blumlein 88 knew a few affordable preamps that I didn't know about.
Of course you can use HPAs as preamps, but unfortunately I don't have the additional inputs.

Preamps are very expensive due to the small number of units, so I use refurbished older NAD preamps, which are really good. However, there are now so many DIY circuit boards, module kits and prefabricated modules that anyone with a little DIY skills can build their own preamp to their specifications.

 

[Hear Here]

In a discussion on one of my previous post the topic of pasive preamp stages came up. This person claimed that they were not ideal and should be avoided.

There really isn't such a thing as a "passive preamp" - it's a misnomer.

The word preamp implies a degree of amplification, normally an increase in the small signal from the source to suit the power amp. They normally include a volume attenuator, so the signal can be adjusated to suit the listener's preference. This may well result in a decrease in signal level compared with what the source offers, but a preamp should always have the ability to amplifier - hence its name.

What you describe is a simple volume attenuator, that may or may not be able to select from a choice of source signals. It cannot amplify so isn't a preamp.

I've never got on well with these "passive volume control" boxes, but others love them for their simplicity and potentially good if built from excellent parts.

 

[norman bates]

Subjective!y to me, maybe due to impedance issues at low volume, a simple pot can sound less dynamic.

 

[Robin L]

Any headphone amp can double duty as a preamp. Quite a few provide the appropriate connectors to make them function as one. Search the review index and you will find plenty of good options.

Almost any. Some headphone amps don't have preamp stages. My Topping E30 does, and I can easily recommend it, along with the updated version, the Topping E30 II.

 

[Roland68]

Almost any. Some headphone amps don't have preamp stages. My Topping E30 does, and I can easily recommend it, along with the updated version, the Topping E30 II.

Neither the Topping E30 nor the E30 II have a preamplifier stage and do not have any components for one.
If you use the volume reduction function of these devices, I recommend taking additional precautions to avoid permanent damage to speakers, headphones and your hearing.

 

[Mikig]

I used a balanced Khozmo for a while and it worked great. If you want absolute silence, the passive is for you. Used with a SMSL DO300 at maximum volume, therefore at its maximum performance, the result was truly excellent.

 

[DVDdoug]

I wouldn't buy an expensive "passive preamp". If you don't need amplification or tone controls it might be a good-simple-cheap solution.

Subjective!y to me, maybe due to impedance issues at low volume, a simple pot can sound less dynamic.

Lower volume is often described as "less dynamic". And in fact it is because there is a greater difference between loud and quiet (or between loud and silent). But there is no dynamic compression.

The impedance effects are minimized at both extremes. At zero volume the output impedance is (about) zero and at maximum volume it's (about) the impedance of the source (with the parallel pot resistance lowering it to some extent).