My passive preamp project – DIY vs Deep pocket

It has been a while since I last took out my soldering iron.

In the last couple of years I have been listening to a Marantz PM8003 integrated Amp. But recently I was attracted to the idea of a passive preamp. (Just a volume control)

From a purist’s perspective, a passive preamp is better because less is more. Removing the active pre stage allows the signal to go directly to the power amp, which means less distortion and colouration.

Before investing more money on a DACT stepped attenuator, TVC and other passive solutions, I decided to test the above hypothesis on my system. So, $50 and 3 hours of my weekend later, I created a simple ALPS pot housing in an aluminium case. (See photos) Nothing much to write home about, simply an RCA socket with decent cable, and one set of input with one set of output. The size of the enclosure is slightly larger, which leaves some room to upgrade to aTVC or DACT attenuator ☺


The most striking improvement from this passive pre is the improved sound stage and transparency. The sound stage was no longer confined to my two speakers and the back wall, it was much wider and deeper; well exceeding my expectations.

The separation also improved noticeably. What used to be a group of musicians, can now easily be defined as an individual player, or rows of players from back to front. It made the whole experience more life like, with the same effect being attained from a well recorded musical. I could tell that each of the performers were moving on the stage as if they were right there.

Compared to the active pre amp it was very transparent There was better coherency which was probably due to less phase shift distortion. This quality made the playback of large scale music so much more enjoyable.

Not all good news

This passive pre is not without short comings. Lower frequency extension is not as good as the active, which is expected.

Not having a remote can be a pain for some people. My DIY pre certainly has no remote control, but it doesn’t concern me.

My passive pre only has one set of inputs. The additional input could be included by adding an input selection knob switch and a larger enclosure.

Due to the lack of active pre stage, I needed to turn up the volume by 20% to obtain the same SPL.

Is it worth it?

You bet! 3 hours and $50 is all it took for a result that would surely have cost me much more if I was to have bought a branded passive.

I may also look into a TVC passive which is suppose to perform better than my cheap ALPS pot.

Please feel free to share your passive pre experience and any suggestions you may have to improve my project ☺

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I tried many sample sounds and the best low budget best sounding is the axiom piece $165w/upgrade a great passive save your cash great sounds dont have to cost big bucks Good luck
You should check out the Intact Audio Slagleman modules if you are considering a DIY TVC project in the future. These are actually autoformers and since you are handy with a soldering iron they should be a snap as they have a circuit board designed by John Chapman that eliminates the excess wiring usually involved in these types of projects.

Try one of these LDR passive pres when they are in stock again. There is a mains lead for the LED's but this is totally decoupled from the signal path.
The LDRs are very good as well. I own one designed by George Stantschleff (Lightspeed Attenuator, go to DIY Audio thread). From a DIY perspective a bit more difficult due to proper matching of the LDR modules. However, if done right they will give any TVC/AVC more than a run for the money.

BTW - there are published schematics on LDRs from George and Nelson Pass. Worth looking into.
Built a TVC using S&B 102's, when they were still available, and before the Promitheus made DIY seem cost ineffective. Was a lot of soldering but not nearly as bad as fitting 46 resistors on a 23 step attenuator. Didn't compare the attenuator/TVC to attenuator/resistors because the latter was for an active preamp upgrade much earlier. Considered a pair of IAG Audio's pre-assembled attenuator/Sowter TVC as an easier alternative. TVC's are indifferent, whether you use balanced (XLR) or single-ended (RCA), which is not the case for AVC's or RVC's.

Still have the TVC but the whole system is optimized for that, with high Vout/ low Zout source with balanced IC's and high gain amps.

The DACT is series, as opposed to ladder, and has more impedance variation than a TVC, but is always additive.

Might want to look at Goldpoint and Marchand attenuators.
Don't let what you built cloud your judgement about transformer or autoformer based passives. They are not even in the same realm as something using an ALPS pot. Try to hear one and you'll most likely agree.

Another way to go is the Pass DIY B1...basically a volume control with an impedance buffer which you can use to match impedance with your equipment.
passive linestages can be GREAT ... I went from a tube pre to a Reference Line Passive years ago and was AMAZED at the difference in sound quality. TVC vs Autoformer vs "normal" passive will always be a debate

Bent Audio has remote kits for some stepped attenuators (DACT and Goldpoint come to mind) which woud cover your remote control concern. Dact and Greyhill switches are good for input selection if you need to add that ... some people also prefer toggle switches vs rotary

you may also desire ground lift toggles if you have any hum issues in your system ... makes you passive more flexible

unless you are maxing out the volume control and still not receiving the desired SPL I would not worry about having to turn up the volume compared to your active pre. I use a passive with a STEP DOWN transformer which allows me to use almost the entire range on my DACT.

Thank you guys for all the good suggestions and ideas.

At this stage my $50 passive is not replacing my active pre due to the lack of bass and lack of remote control. Certain music I will listen with this passive other times I will stick the active stage.

TVC and AVC are on my list of consideration and impedance variation seems to be an important issue. What is that? I use a 50K pot in my project. and that Pass DIY B1 buffer what is that?

Sorry for all the questions I am ok with soldering but I am not very good with all the electronic stuff.
Nice project. simple clean and efffective! thanks for the affect and I really enjoy reading it.
>>and impedance variation seems to be an important issue. What is that? I use a 50K pot in my project. and that Pass DIY B1 buffer what is that? <<

Pots and ladder attenuators present a constant input impedance (Zin), varying by frequency (duh). Series attenuators will never present less than device following it. I've preferred series attenuators for specific "tuning" applications but haven't used one as a master volume control, yet. TVC's and AVC's... I'll just direct you to, which was well documented, for an example.

Lowest possible Zout and highest possible Zin would be the theoretical ideal but reality is never ideal. Many other factors. Tubes tend to have a higher Zout but not usually an issue. When there is an impedance mismatch, both low and high frequency gets rolled off. As I type this, it occurs that capacitance and inductance becomes the real issue. If you've looked at a passive xover, you'll follow my logic.

Choosing a Zin value for a pre-assembled attenuator, which is a relatively recent option, is system dependant.

A buffer is an active stage that has little or no gain. Primarily used for impedance benefits. Placette Active is another example. Musical Fidelity and Space Tech make tube buffers.

One difference between RVC's an TVC's is that the former attenuates to heat and the latter converts to amperage. What that means, I have no idea. When I built my TVC, knowing (or thinking I knew) the difficulties with xformers, I didn't expect it to work well. The chief designer for a respected audio brand (my brother) sat in my living room and told me they wouldn't work and why until I pointed out that he was praising one minutes earlier. Even John Chapman of Bent Audio told me he had to be convinced at first. 'Chatta', as you have learned, any passive might not work in every system.
Chatta, a volume control is simply a device that "dumps" (resists the flow of) energy, in varying amounts depending on where you turn the pot...

The reason you are missing low frequency energy (and you have more hi frequency: details, etc) is that the input of yr amp & the output of yr pot don't get along that well.
As a rule of the thumb, you need two things for a great sound with a passive:
a) 1/10 to 1/1 impedance ratio, constant, between output & input (amp).
b) Good energy output from the component producing the sound (i.e. the source).
Hi Ngjockey,

Thank you for all the great info! I am aware of the MF buffer years ago but never read into the subject then.

Based on your suggestion it seems like my pot is connected in series. Is that right?

Someone suggested a shunt type but it has that very annoying sound leakage at lower levels if you know what I mean.

I understand that a pure passive is hard to match to a lot of systems. I read that speaker sensitivity also plays an important role in the whole issue. And that is where the buffer stage comes in. According to my googling, pro-audio use buffers extensively to match different stages and resolve long cable run issues. An audio buffer stage is supposed to provide little to no gain and hence minimal colouration.

My thinking at the moment is to add a unity gain buffer stage behind my pot or stepped attenuator. Which basically turns my project into a low gain pre amp In particular I am looking at this Australian made audio buffer from Burson Audio.

What do you think of that idea. Also what is the sonic different between a stepped attenuator to a pot?

The "series" I was referring to is not the same as the link but not worth getting into for your purpose.

Usually, buffers are placed before passive, as demonstrated on the Burson website. Although, with a 50k pot, you might be right.

Best way to answer last question: Yes. Once you've tried an attenuator, you'll never want a pot again.
Should also point out that many sources double Vout when using XLR outputs and Zin can double as well with balanced inputs. Can be impractical for additional parts required, except with TVC.
pots or stepped are fine but passive with transformers are much better.
no problems with impedance matching, more body, better soundstage.
tvc is your next step.
then music first audio.
and finnaly ypsilon electronics passive/active preamp.