Passive Preamp

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I am a newbie and I have a few questions for Harry or anyone else who can answer.

1. How does this pot with the shunt or whatever compare to a stepped attenuator like the one from Foreplay?
2. What exactly do I need to add another input and hopefully another output? I think I'll need some kind of switch for the input, but what is a good part for price/performance?
3. If I am using ASL Wave 8's and am worried about them driving my speakers, will a passive have enough gain to get the most out of the amp? I have an Art Dio with a hot, 7 volt output so I think I am perfectly set up for a passive pre, but low-powered tube monos with my 89 db efficiency Sound Dynamics RTS3 make me wonder how it'll all work.

Thanks,
Ryan
 
I am sure Harry could give you more than enough info about what to do but I am not familiar with Foreplay but I don't have a step attenuator at the moment so I can't really do a fair comparison. But I remember Harry mentioning some where in this thread about, How he would put a ALPS BB pot and high quality resistors again any step attenuator.
Yes, all you would need is a DPDT switch for 2 input pair of connectors hopefully the switch will have a center off position for better isolation but you wouldn't need a switch for two outputs just run them in series and maybe a mute switch for both.
What is a ASL Wave 8?

😎
 
I cant comment as to if you will have enough total gain in the system to adequately drive your speakers but i can make a couple of general comments.

7V is very high for line level 😀

passive preamps have no gain whatsoever only cut (attenuation)

stereo pots have poor tracking between the channels and less than ideal channel separation

log pots are far from providing a log transfer fuction

using a linear pot and a resistor to attain a quasi log transfer function will produce a linear region that is too small to provide enough attenuation control to be useful to most people using high end equipment

using a stepped attenuator, a very accurate transfer function may be attained providing that it allows enough 'steps'
 
FBJ,
I just noticed I had a typo. I meant the stepped attenuator from Bottlehead called Sweet Whispers I think. I had heard stepped attenuators were generally better than pots, but I myself have no idea.

What does DPDT stand for? I would also appreciate a recommendation for what exact switch to use.

The ASL Wave 8 is a 8wpc tube monoblock from Antique Sound Labs. They are supposedly great for the price and can be modded to sound much better. They go for $99 each, or $119 with a cage to cover the tubes. I should get mine sometime this week.

AudioFreak,
I don't know the quality difference between a stepped attenuator and the shunted dual mono pots that Harry is using, but I would like to make the best inexpensive pre while still being functional. I will have two sources, one of which has the extremely hot output and one from my vcr which should be much more typical, so I will need a decent range so I can have enough volume control for both inputs.
 
Musiclover:
Regardlees of which player you use, at full volume with a passive preamp, your voltage in will equal your voltage out. You can plug a cd player direct into an amp and it will drive the amp to almost rated power. If 7 volts is too much then cut some of it out by turning the volume nob down. As far as DPDT (double pole double throw) this is the kind of switch you need for your selector. It is actually two sets of seperate contacts (one for each channel) within the same switch. I use Grayhill which are about 20.00 for a six pole double throw, but I get room for six seperate inputs. You can use any type of switch as long as it is at least 2 pole and double throw.
The big difference between a stepped att. and a shunt passive is the resistor in the signal path. As resistors are a source of noise you can't really get any quieter than a straight wire. It's a valid argument for both sides. Harrys passive can be built cheaply and upgraded as needed. You can run down to radio shack and build a quikie for around 10.00 and see if you think it will do what you need. If it works then spend away using the parts you really want. Good luck and have fun.😉
 
MusicLover4-
PassFan said about all I would. But If you still need help maybe if you e-mail Harry direct and be real nice (say you like his jokes) he can send you a schematic and part numbers of the radio shack parts. His is by for cheaper than mine and as for the silver wire he used just use regular wire untill you can order what you want.
The people that work at radio shack are not to bad on simple parts and how to use them. I have some old radio shack how to books I brought back in the early '80s. These books may help you too.
😎
 
PassFan said:
Musiclover:
Regardlees of which player you use, at full volume with a passive preamp, your voltage in will equal your voltage out.
Click to expand...

Well many passive preamps are wired in such a manner than even @ full volume, they will still have a certain fixed amount of attenuation that is inherent to the design.

Nevertheless, shunt attenuators can be very nice.
 
Yes, I agree. I didn't want to give an overload as it seemed he was stuck on getting gain out of a passive preamp. I tried stick to a laymans explanation to make it obvious. Just for kicks I've measured volume pots anywhere from 0 ohms to .3 ohms. Sometimes they're not as accurate as we think. If Musiclover takes my advice and tries a cheap one out first all his doubts will be erased and he will know either way. Thanks

P.S. Audiofreak: I kind of expected to be called on the noisy resistor thing to tell you the truth. You took me by surprise.😀
 
volume pot topology

At first sight, this topology looks superior: the signal passes through a good solder connection, the pot just shunts unwanted energy to ground, whatever it does is not what you hear....

But remember that it is still a voltage divider, and at lower volumes, the output voltage and its "errors" will be dominated by what happens in the pot and its connectors.

Eric
 
Ok, I have a working passive preamp. I followed Harry's design very closely, with two 22k resistors in parallel, the 100k audio taper pots from radioshack, and some cheap 30 guage wire with the insulation removed, and a cheap 8 phono input board all from Radioshack. I am also using a dpdt switch to have two inputs, a vcr and an Art Dio.

It sounds pretty good, but I have a few questions/comments.

1. It has some hum, is there any way to get rid of it?It isn't really noticeable when music is playing, but it is still there.

2. The volume pot seems useless. I am trying to figure out the best way to use this vol. control, but the preamp is basically impossible to change. The slightest change either results in the sound disappearing or becoming extremely loud. It is like this with both inputs, my vcr and my 7 volt Art Dio. I would like a little flexibility with the pot, but I do have a digital volume control on my cd player and my pc, which both go to my Art Dio, so it would be alright to level match to the vcr and then change. I would like to know if I should think about using different resistors or anything to better maximize sound quality and/or functionality, since I have a slightly different use than some with my odd output of my dac and the use of a vcr too.

3. What would be the things to upgrade, where do I get the parts, and how much do they cost?I have heard to use caddock or vishnay resistors instead, silver wire, and Alps Black Beauty pot. I would think about the silver wire and Alps Black Beauty pots first, but any ideas would be welcome. Also, are the Black Beauty pots mono, because stereo would be easier if it were as good for the money.

Thanks for any help with all of my questions.

Ryan
 
If you don't feel comfortable with a range of control the preamp gives you try to use different values for 10K series resistors. I'm using 24K resistors as they work the best in my setup. Also check if your pot is logarythmic (and not linear).
Depending on the value of a series resistors you would get different attenuation at maximum volume position.
By all means feel free to use stereo pot as it is much easier to change the volume than doing it with two separate pots and having to adjust the balance each time. I don't think that using two separate pots is very convenint in everyday listening.
If you think about upgrades change the series resistors first as they influence the sound the most. I would try Holcos or Rodenstein first because for the price you can't beat them. I preferred Holcos in some locations over the Caddocks.
 
Alright, I am glad you started something. More members in this forum are willing to give you help if the person asking has actually built something. HPotter is right. You going to have to play with the 22k resistors in value up or down. Use some cheap resistors first to get the volume lever you want from min to max rotation.
As for the hum problem I think you must have something hooked up wrong or your bare wires might be touching some where.
Are the inputs and output connectors have insulated washers?
Do the inputs ground go directly to the pot? If so, then the two right inputs ground tabs should be in series and should have a wire going to the ground lug of the pot and that should have another wire from that lug to the output connector ground tab. And the left connectors should be tied together the same way.
Only the right and left center connections of the input connectors should be connected to the DPDT switch. Input one left and right to the top part of the switch. Input two to the bottom part of the switch and the center of the switch will be connected to the pot left and right lugs.
I have some of the Radio Shack 100k pots for a headphone amp. I have been moving and I my stuff is across town and I won't have time to move any of it untill this weekend.
But the pots are audio taper, I sure of it.
 
i had typed up a nice long post here, but i closed the window by mistake. ugh.

anyway, i think i will build one of these puppies. i have a ghetto 10k alps pot in a plastic radio shack project box as my "passive pre" right now, it is not even in shunt configuration but it still manages to sound better than the active preamps i've tried, both tube and solid state. so i may as well get off my butt and build a proper one. seeing all those nifty pictures from HPotter has really whetted my appetite.

some construction details: i've been using raw Cardas litz wire (99.9997% OFC w/polyeurethane varnish, same wire used in all Cardas' designs) in the 30-gauge size with great success, i think it makes a great hookup wire whether paralelled or single-strand. i need input selection, so i will probably use some Omron SPST relays for switching, using a rotary selector switch and an external "wall-wart" transformer to power the relays. i have been using 2% silver bearing solder or wonder solder, but i'll try the 4% variety if you all say it sounds better. i'll probably go with a 10k series resistor (holco or roederstein or maybe even a Mills wirewound, too cheap for vishay/caddock which may or may not sound better anyway), along with a 20k or so Noble stereo pot (i hate adjusting dual mono controls). the RC input filtering is an interesting idea, i might give it a shot... maybe make it switchable so i can see if it sounds better or not? as for the case... probably an aluminum or plastic project box, lined with damping material and double-sided PCBs for extra copper shielding.

incidentally, using a 10k series resistor is not so bad. most of the time you will be listening with the volume turned up quite a bit, where it will have some resistance... e.g at -12dB attenuation it'l be at 3.3k, which makes the input impedance 13.3k. combined with say a 50k input impedance amp, that makes total load seen by the source about 13k. not so bad. so i would err on the lower side with the overall impedance of the preamp, to minimize noise/signal loss, unless your source component has an especially weak output section.

ok, time to order some parts...

cheers,
dorkus
 
More pictures for your apetite

Thats the preamp I'm using. The mid, high section is passive; subwwofer part is active. Active section is based on A75 input stage. It's potted and each channel has 10 relays which change the feedback resistor to set gain. The volume control is shunt type with 24K series Holco resistor and remotely contolled Alps 100K potentiometer. I have more inputs that I'm actually using.
 

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Hi,

I just built one myself, following the schematic planet10 posted. But (there is always a but), I used a linear pot instead of a logaritmic, so there is a _very_ small low level part of the pot when I use it.

It worked really well for what I use it for.

Isn't it some way to make a pot semi logaritmic connecting an extra resistor somewhere?

//magnus
 
Thanks Dave,

I added a resistor to my _very_ low quality pot, and it gave me a little bit more room to play low level volume.

I'm very satisfied. Just as many others in this thread, I'm sure this is the most bang for the buck DIY I've ever attempted. My Leach Amp liked it just as good as any other pre amp, so far.

My Alps pot will arrive tomorrow.

//magnus
 
Randy

Yes, control unit would be a better term.

For the benefit of those who's education has been limited or for whom English is not a first language, an oxymoron is a figure of speech by means of which contradictory terms are combined so as to form an expressive phrase or epithet.

Geoff
 
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