Reccommendations for a preamp build based on my gain and impedance needs
Hi all. I recently bought a partially working Quad 306 power amplifier, had to replace a few components, and whilst I was at it I changed the input sensitivity to 1V. I've tried it with the pre-out of a Schiit Vali 2 headphone amp with my main speaker, and it sounds pretty good and probably better than my main integrated amplifier which is a NAD C316BEE.
So, I'm in need of a preamp. I have built a Bottlehead Crack headphone amp kit before, so I was thinking of possibly ordering and making their Moreplay preamp. However, I have seen many other preamp builds on here and elsewhere that have caught my eye. Such as: DIY ECC802S (12AU7 / ECC82) Vacuum Tube SRPP Preamplifier and Low-mu preamp
I would be using it with my Trichord Dino MkII with Dino+ PS. Am I correct in thinking that because of the switchable loading capacitance, resistance and gain this makes this phono stage more flexible to fit into a setup?
My knowledge of electrical circuits and things such as the input/output impedance of amplifiers is VERY basic, but I'd like to learn. Because of this, I would love to get some advice or to be pointed in the right direction with this build.
My requirements for the preamp is 2 or 3 unbalanced selectable inputs and no phono stage needed. Possibly remote-controlled volume, but only if it makes sense in the build and isn't too complicated. I would like to make a tube preamp, and probably one that adds some 'tube sound', as I've got a feeling it might work well with the Quad 306.
With regards to gain and input/output impedance requirements, this is where I need some help, as I don't really know where to start.
I have tried to do some research myself on what my requirements are, and I hope this doesn't come across as a 'recommend me the best kit/build' type of thread with no self-research done, but I'm just a bit overwhelmed with all the information.
Thank you in advance for any help!
Chris
Hi all. I recently bought a partially working Quad 306 power amplifier, had to replace a few components, and whilst I was at it I changed the input sensitivity to 1V. I've tried it with the pre-out of a Schiit Vali 2 headphone amp with my main speaker, and it sounds pretty good and probably better than my main integrated amplifier which is a NAD C316BEE.
So, I'm in need of a preamp. I have built a Bottlehead Crack headphone amp kit before, so I was thinking of possibly ordering and making their Moreplay preamp. However, I have seen many other preamp builds on here and elsewhere that have caught my eye. Such as: DIY ECC802S (12AU7 / ECC82) Vacuum Tube SRPP Preamplifier and Low-mu preamp
I would be using it with my Trichord Dino MkII with Dino+ PS. Am I correct in thinking that because of the switchable loading capacitance, resistance and gain this makes this phono stage more flexible to fit into a setup?
My knowledge of electrical circuits and things such as the input/output impedance of amplifiers is VERY basic, but I'd like to learn. Because of this, I would love to get some advice or to be pointed in the right direction with this build.
My requirements for the preamp is 2 or 3 unbalanced selectable inputs and no phono stage needed. Possibly remote-controlled volume, but only if it makes sense in the build and isn't too complicated. I would like to make a tube preamp, and probably one that adds some 'tube sound', as I've got a feeling it might work well with the Quad 306.
With regards to gain and input/output impedance requirements, this is where I need some help, as I don't really know where to start.
I have tried to do some research myself on what my requirements are, and I hope this doesn't come across as a 'recommend me the best kit/build' type of thread with no self-research done, but I'm just a bit overwhelmed with all the information.
Thank you in advance for any help!
Chris
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Ah, I never thought of doing that. So, would I just use a 1/4inch to RCA adapter in the headphone socket?
Unfortunately, I use the Crack at my desk, and not at my main setup. Also, I'd still like to get my hands dirty and build something new, as I really enjoyed making the crack.
Unfortunately, I use the Crack at my desk, and not at my main setup. Also, I'd still like to get my hands dirty and build something new, as I really enjoyed making the crack.
By 'input sensitivity 1V' do you mean the amp now gets to full power out with 1V rms signal input? If so, you can't need very much gain. Maybe you need no gain at all...
The Quad 306 is an older solid state amp, so it probably has an input impedance of about 50k ohms, but maybe as low as 10k ohms.
If you make some kind of line stage with a (low output impedance) cathode follower or MOSFET source follower output buffer, you won't have to worry about impedance issues.
You could use a headphone amp like your Crack as a line stage. You won't need to sink anywhere near that much current into the power amp input, but I suppose it wouldn't hurt to have it on tap. Just be careful you never unplug/replug things with the preamp and amp powered on! You don't want to send high signal voltages to the power amp inputs.
The problem will be how to create a line stage with gain of only about 2X. Even low mu triodes will have gain of 4X to 10X.
Perhaps a plain single 12B4A per channel would work?
12B4 Preamp
Or a plain single 6V6 triode wired per channel?
6V6 line preamp
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The Quad 306 is an older solid state amp, so it probably has an input impedance of about 50k ohms, but maybe as low as 10k ohms.
If you make some kind of line stage with a (low output impedance) cathode follower or MOSFET source follower output buffer, you won't have to worry about impedance issues.
You could use a headphone amp like your Crack as a line stage. You won't need to sink anywhere near that much current into the power amp input, but I suppose it wouldn't hurt to have it on tap. Just be careful you never unplug/replug things with the preamp and amp powered on! You don't want to send high signal voltages to the power amp inputs.
The problem will be how to create a line stage with gain of only about 2X. Even low mu triodes will have gain of 4X to 10X.
Perhaps a plain single 12B4A per channel would work?
12B4 Preamp
Or a plain single 6V6 triode wired per channel?
6V6 line preamp
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In the Quad 306 service manual, it says "Signal input level: 0.375V for 50W into 8ohms. Amplifier loads the input by 20k ohms". However, I have changed this to 1V.
I'm not quite sure if that is what you're saying, but I change it from 0.375V to 1V so it might get on a bit better with whatever I pair with it.
Where it says, "Amp loads the input by 20k ohms", does this mean its input impedance is 20k ohms?
The manual also states, "Output internal impedance and offset: 1.5uH in series with 0.05ohms. Offset typically 7mV."
I have tried the crack with it, but even turning the volume knob up a few percent, it was already too loud. I was surprised by this because I thought by changing the input sensitivity I would be able to increase the volume on the pre more.
Also, I'm not stuck with that input sensitivity, I can always change it again quite easily.
Thanks for pointing out those two plans, I'm going to do a bit of research on them.
I'm not quite sure if that is what you're saying, but I change it from 0.375V to 1V so it might get on a bit better with whatever I pair with it.
Where it says, "Amp loads the input by 20k ohms", does this mean its input impedance is 20k ohms?
The manual also states, "Output internal impedance and offset: 1.5uH in series with 0.05ohms. Offset typically 7mV."
I have tried the crack with it, but even turning the volume knob up a few percent, it was already too loud. I was surprised by this because I thought by changing the input sensitivity I would be able to increase the volume on the pre more.
Also, I'm not stuck with that input sensitivity, I can always change it again quite easily.
Thanks for pointing out those two plans, I'm going to do a bit of research on them.
Usually, the spec "input sensitivity" means "signal voltage required to drive the amplifier to clipping/maximum output". By changing the input sensitivity from 0.375V to 1V, you should have made the amp less sensitive (requiring more signal voltage to drive the amp to full power).
Yes, it does look like the input impedance of that amp is 20k ohms. You'll want to make sure the output impedance of your preamp ends up being 2k ohms or less. The lower the better, but 2k is what's actually required (10X lower than the load impedance).
Output impedance of an amplifier is the source impedance presented to the load (the speaker). That spec is saying the output has inductance of 1.5uH and an output resistance of 0.05 ohms. But that has little to do with the choice of preamp.
If you drive the power amp with a DAC or other digital player, those are standardized to play a maximum of 2V rms (2.828V peak) signal level at full scale (0dBFS). 2V rms is the maximum signal out from a DAC or other digital player. The nominal 'average' level is 0.2V rms (200mV rms). Peaks will be 10X higher than that, so 2V max.
A digital player sending 2V peaks to the power amp will send it +6dB (2X) past clipping (which happens at 1V input).
Since the DAC provides all the signal out needed to drive the amp well past full volume, you actually do not need a preamp at all.
Since the Bottlehead Crack has a gain of about 3X or so (10dB), it is amplifying that 200mV signal to something like 600mV, and 10dB (3X) peaks are going to 6V out. The amp is getting sent far into overload that way. So you have to turn the volume control way down.
You are finding out that you don't need a preamp with gain.
Since you don't need any gain, you have to ask yourself what it is exactly that you want your 'preamp' to do.
Add 'tube sound'? But what exactly is that? Harmonic distortion of a certain kind? If it is, then what kind, exactly?
??
Yes, it does look like the input impedance of that amp is 20k ohms. You'll want to make sure the output impedance of your preamp ends up being 2k ohms or less. The lower the better, but 2k is what's actually required (10X lower than the load impedance).
Output impedance of an amplifier is the source impedance presented to the load (the speaker). That spec is saying the output has inductance of 1.5uH and an output resistance of 0.05 ohms. But that has little to do with the choice of preamp.
If you drive the power amp with a DAC or other digital player, those are standardized to play a maximum of 2V rms (2.828V peak) signal level at full scale (0dBFS). 2V rms is the maximum signal out from a DAC or other digital player. The nominal 'average' level is 0.2V rms (200mV rms). Peaks will be 10X higher than that, so 2V max.
A digital player sending 2V peaks to the power amp will send it +6dB (2X) past clipping (which happens at 1V input).
Since the DAC provides all the signal out needed to drive the amp well past full volume, you actually do not need a preamp at all.
Since the Bottlehead Crack has a gain of about 3X or so (10dB), it is amplifying that 200mV signal to something like 600mV, and 10dB (3X) peaks are going to 6V out. The amp is getting sent far into overload that way. So you have to turn the volume control way down.
You are finding out that you don't need a preamp with gain.
Since you don't need any gain, you have to ask yourself what it is exactly that you want your 'preamp' to do.
Add 'tube sound'? But what exactly is that? Harmonic distortion of a certain kind? If it is, then what kind, exactly?
??
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Thank you for explaining those few things, it's really helpful!
I have mainly been using the Quad with a Schiit Vali 2 headphone amp with a pre-out. Am I correct in thinking that this pre-out is acting as a passive pre? Hence why I can crank up the volume a lot more with it.
I mainly use a turntable with a Trichord Dino phono stage, and I've been trying to find out what the output voltage of the phono stage is. I understand the basic principle of a phono stage, and I know that with the settings I have on the Dino, it's at 48dB gain and my carts output is 3.5mV, is it possible to calculate the output voltage of the phono stage with this? Or would I need to measure it somehow? Could I play a test signal through the phono stage then measure the output with a multimeter? Or does this get complicated because it won't be consistent like how you explained with the digital source peaking at 2V but nominal avg. 0.2V rms?
When I say 'tube sound' I'm probably just regurgitating erroneous information I've read elsewhere, I'm not actually fully certain what I mean hah. I know that tube sound means distortion of some kind, but not exactly what kind. I will have to do some research on that.
If my preamp in this scenario doesn't need any gain, is there much point in building a tube one rather than a SS one? Would I be correct in thinking that 'tube sound' or harmonic distortion is added through gain? I'm not completely fixed on making a tube pre, if a SS pre would make more sense I could go with that.
So, I guess what I need for this setup is really just a box with volume control and input selection?
To help with my searches, what's the best way to describe a simple pre like this that doesn't add gain? Is it just a passive pre?
Sorry about all the questions, and thank you for your help, it's really appreciated.
I have mainly been using the Quad with a Schiit Vali 2 headphone amp with a pre-out. Am I correct in thinking that this pre-out is acting as a passive pre? Hence why I can crank up the volume a lot more with it.
I mainly use a turntable with a Trichord Dino phono stage, and I've been trying to find out what the output voltage of the phono stage is. I understand the basic principle of a phono stage, and I know that with the settings I have on the Dino, it's at 48dB gain and my carts output is 3.5mV, is it possible to calculate the output voltage of the phono stage with this? Or would I need to measure it somehow? Could I play a test signal through the phono stage then measure the output with a multimeter? Or does this get complicated because it won't be consistent like how you explained with the digital source peaking at 2V but nominal avg. 0.2V rms?
When I say 'tube sound' I'm probably just regurgitating erroneous information I've read elsewhere, I'm not actually fully certain what I mean hah. I know that tube sound means distortion of some kind, but not exactly what kind. I will have to do some research on that.
If my preamp in this scenario doesn't need any gain, is there much point in building a tube one rather than a SS one? Would I be correct in thinking that 'tube sound' or harmonic distortion is added through gain? I'm not completely fixed on making a tube pre, if a SS pre would make more sense I could go with that.
So, I guess what I need for this setup is really just a box with volume control and input selection?
To help with my searches, what's the best way to describe a simple pre like this that doesn't add gain? Is it just a passive pre?
Sorry about all the questions, and thank you for your help, it's really appreciated.
SY's heretical preamp has less than unity gain and very low output impedance, but you won't get 'tube sound' from it.
12AU7 srpp a solid choice.
I have built the SRPP line stage, and it came out good enough I've built 6 of them now. First one I built with parts on hand, the rest got built with a phonostage in front of them. The build page has recommended a 3 input selector and remote volume control. I didn't go that route, mine is all manual. It sounds absolutely amazing, and can drive a wide variety of power amps.
Check out my thread on Audiokarma titled "Members Only preamp" theres pictures and a full description of just what the heck I was thinking...
I have built the SRPP line stage, and it came out good enough I've built 6 of them now. First one I built with parts on hand, the rest got built with a phonostage in front of them. The build page has recommended a 3 input selector and remote volume control. I didn't go that route, mine is all manual. It sounds absolutely amazing, and can drive a wide variety of power amps.
Check out my thread on Audiokarma titled "Members Only preamp" theres pictures and a full description of just what the heck I was thinking...
A 12AU7 SRPP is going to have a gain of about 8X, more or less.
As is often the case, there's no gain needed here. Therein lies the problem...
As is often the case, there's no gain needed here. Therein lies the problem...
Schiit Vali 2 has a gain switch. If you have it set to LOW, then the gain = 1, which means it does not provide any gain at all. If that's how you're using it, then everything you've been saying makes sense.
Do you find the Vali 2 adds anything to the sound?
You can get close enough.
If at 1kHz the nominal level of the cartridge is 3.5mV (0.0035V), and the preamp supplies 48dB gain (250X), then .0035V * 250X = 0.875V output from the phono preamp with that input level.
On a peak of say +10dB (3.2X gain) from that nominal level, the max signal voltage out will be 2.8V. That will drive the power amp well past clipping. So the answer to the question is, no you do not need a line preamp with gain from your phono preamp.
However, we need to consider what the output impedance of your phono preamp is, and whether it can drive the cables and any input switching and volume control downstream, which adds capacitance to the input capacitance of the power amp.
The Trichord Dino web page is pretty skimpy with specifications, but it does say it employs an "analogue output stage using OPA827 op amps". That means the phono preamp probably can drive a reasonable length of cable, selector switch, volume control, and load presented by the power amp input circuitry.
Once again, the answer is that you do not actually need a line preamp in your setup. What you want might be different, though... 😉
I will readily admit that I usually prefer a little bit of 'tubiness' to the sound. I think it comes from adding a bit of low order harmonic distortion. Before I finished a tube phono preamp recently, I was making do with a Hagerman Bugle preamp. Adding a 6DJ8 anode follower line stage with about 2X gain fattened up its sound to my liking. What's interesting is that I did not like the added tubiness if I added that tube line stage after my Topping DAC. It thickened the sound and made it a bit too muddy for me. But it did help the opamp-based phono preamp. It's purely a taste thing. Nothing to do with 'true high fidelity playback'. Just what I happen to like for my own enjoyment...
No, a buffer (FET 'source follower' or tube 'cathode follower') can be configured to add some distortion, but slightly less than unity gain.
Yup. A control box.
That's what a lot of people call it. A 'passive preamp' (which makes no sense since there's no 'amp' involved, but that's what people call it...).
My pleasure!
You might consider, as a fun little project, building a simple cathode follower out of a common (and hopefully cheap) twin triode like a 12AU7, 12AT7, or similar. These are guitar amp tubes you can buy at most musical instrument stores, or find cheaply on eBay, etc. There are other twin triodes that would work as well, such as Russian types like 6N1P, 6N3P, 6N6P. These are usually inexpensive.
A cathode follower will provide just a little less than unity gain, usually about 0.95X to 0.97X. That's not enough of a loss for you to hear.
You can bring out a more 'tubey' flavor to the sound (more tube distortion) by configuring the cathode follower with a low plate voltage and low plate current. Be prepared to experiment to get the sound to your liking, or you may find that adding this does nothing to improve the sound. But you will learn some things along the way, which you may find interesting -- maybe even fun.
Look here at member MerlinB's page on AC-coupled cathode followers, which culminates in a section about the "Fixed Bias Cathode Follower".
The Valve Wizard -Cathode Follower
That shows an example of a fixed bias cathode follower using a 12AU7 (ECC82) that can be built as a standalone buffer. It will have gain of about 0.95X, so would work in your setup.
Thank you for the detailed reply! Yes, I have been using the Vali set to low gain. I'm not sure if it adds anything to the sound of the Quad, because I haven't used it properly with anything else. When I hooked up the crack there was a lot of hum (possible ground loop), and I haven't tested my other headphone amp with pre-out with it yet.
I did find a bit more info on the Dino's output voltage, even though you've shown how to calculate it, but here it is anyway: "RIAA reproduction characteristics, Output voltage: 0.5V at 1KHz"
So, it looks like I'm going to research and hopefully build a cathode follower of some kind! Thank you for all your advice and explanations, you've been incredibly helpful!
I did find a bit more info on the Dino's output voltage, even though you've shown how to calculate it, but here it is anyway: "RIAA reproduction characteristics, Output voltage: 0.5V at 1KHz"
So, it looks like I'm going to research and hopefully build a cathode follower of some kind! Thank you for all your advice and explanations, you've been incredibly helpful!
I just took a look, and that looks fantastic, but as Ron pointed out that might not be a good fit for the setup in question. I'm going to keep that bookmarked for inspiration in the future, though. Thanks
I just remembered that there are Glassware PCBs to make standalone cathode followers:
Tube-Based Buffers
The 12VAC Aikido Cathode Follower would be easy to put together, since it uses a 12VAC wallwart power supply for both the tube heaters and the B+.
ACF 12Vac Tube Stereo Buffer
If you want to go the more traditional route, there are other PCBs that use various kinds of tubes.
Tube-Based Buffers
The 12VAC Aikido Cathode Follower would be easy to put together, since it uses a 12VAC wallwart power supply for both the tube heaters and the B+.
ACF 12Vac Tube Stereo Buffer
If you want to go the more traditional route, there are other PCBs that use various kinds of tubes.
Ah, I was just about to ask about Glassware's CFs. I think I would quite like to build my own power supply for whatever I build (so I get to do more, and to reap any sonic benefits if there are any), so, would any of their other cathode followers be a better fit?
Their All-in-One tube buffer (ACF-2 Aikido Cathode Follower Kit) for example states it's for those that need a unity gain buffer (which I know I need), but also those that need more current than a passive line stage. How could I work out my current needs? I've had a look online regarding this question, but I'm struggling to find anything out.
I suppose my question is, why would one want more current than a passive line stage?
I have also seen it is fairly cheap and straightforward to have custom PCBs printed and made. Have you had any experience doing this?
Their All-in-One tube buffer (ACF-2 Aikido Cathode Follower Kit) for example states it's for those that need a unity gain buffer (which I know I need), but also those that need more current than a passive line stage. How could I work out my current needs? I've had a look online regarding this question, but I'm struggling to find anything out.
I suppose my question is, why would one want more current than a passive line stage?
I have also seen it is fairly cheap and straightforward to have custom PCBs printed and made. Have you had any experience doing this?
Re: current -
The source (cathode follower/preamp) has to drive the input impedance (resistance, capacitance and/or inductance) of the load (power amp input circuit) at all frequencies from below 20Hz to above 20kHz.
A potentiometer and selector switch (a 'passive preamp') does not sink any current at all (they are 'passive'). Therefore, they add to the load on the source (the phono preamp output, DAC output, etc.). Now the source has to drive both the amplifier input circuit and the passive selector switch and volume control. The 'passive preamp' makes the load *more* difficult to drive, not less.
The cathode follower is a buffer, so will provide extremely high input impedance and low output impedance. A typical 12AU7 cathode follower will have roughly 5mA of plate current, so can sink ('deliver') that 5mA into the load. That's a good thing if the load has appreciable capacitance, such as you might see if you have a 6 or 10 foot interconnect cable going to the input of a solid state power amplifier. The combined cable+input capacitance could be as high as 2000pF. (Not likely, but it's possible. I have one long RCA cable that measures 1500pF, all by itself.) It's at high frequencies where this can be an issue, due to slew limiting. The high frequency signal fluctuates very quickly (20,000 times a second) and the source has to charge that load capacitance that quickly. If it can't (i.e. if the source current is too low) then a high frequency sine wave signal can be distorted into something like a triangle wave, indicating slew limiting ('slewing distortion'). This is not a likely problem, but it is possible.
Re: custom PCBs -
There's a learning curve, but yes, that's an option. Express PCB and Sprint Layout are two possible softwares available for this. There are several PCB fabs in China that make small quantities of custom PCBs for cheap. You'll need to learn about gerber files and all that.
The GlassWare PCBs are very good. Stout, well made, and well thought out. I don't think you can go wrong with those.
Member kodabmx has a selection of tube project PCBs for sale too.
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The source (cathode follower/preamp) has to drive the input impedance (resistance, capacitance and/or inductance) of the load (power amp input circuit) at all frequencies from below 20Hz to above 20kHz.
A potentiometer and selector switch (a 'passive preamp') does not sink any current at all (they are 'passive'). Therefore, they add to the load on the source (the phono preamp output, DAC output, etc.). Now the source has to drive both the amplifier input circuit and the passive selector switch and volume control. The 'passive preamp' makes the load *more* difficult to drive, not less.
The cathode follower is a buffer, so will provide extremely high input impedance and low output impedance. A typical 12AU7 cathode follower will have roughly 5mA of plate current, so can sink ('deliver') that 5mA into the load. That's a good thing if the load has appreciable capacitance, such as you might see if you have a 6 or 10 foot interconnect cable going to the input of a solid state power amplifier. The combined cable+input capacitance could be as high as 2000pF. (Not likely, but it's possible. I have one long RCA cable that measures 1500pF, all by itself.) It's at high frequencies where this can be an issue, due to slew limiting. The high frequency signal fluctuates very quickly (20,000 times a second) and the source has to charge that load capacitance that quickly. If it can't (i.e. if the source current is too low) then a high frequency sine wave signal can be distorted into something like a triangle wave, indicating slew limiting ('slewing distortion'). This is not a likely problem, but it is possible.
Re: custom PCBs -
There's a learning curve, but yes, that's an option. Express PCB and Sprint Layout are two possible softwares available for this. There are several PCB fabs in China that make small quantities of custom PCBs for cheap. You'll need to learn about gerber files and all that.
The GlassWare PCBs are very good. Stout, well made, and well thought out. I don't think you can go wrong with those.
Member kodabmx has a selection of tube project PCBs for sale too.
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Perfect, thank you. Well, I think I'll go with their ACF-2, so I just need to now work out whether to get the octal or 9-pin version and also what power supply to go with it.