Hello guys,
I recently built a preamp and has some noise issue. It is not like hum, buzz, or hiss but sounds very much like white noise. Then I listened to some suggestions, reworked some parts and changed some chokes, filament supply, etc. None of them really worked for me, but reducing gain definitely helped. I tried adding a 1K resistor with a 100uF cap paralleled to the plate resistor for 6SN7 (39K at R7/R8). The white noise is completely gone. However, by doing so, the sound becomes quite weird. Sounds very analytic and the black magic of its midrange is totally gone. So I have been seeking for a way to reduce the gain and keep the original signature of the sound and might improve it if possible.
Here is a way that my friend suggested me to do, please see the attachment. We want to have about 8-10mA plate current and about 15dB gain so I would have sufficient gain and have 6SN7 to work at its sweet spot -- to my understanding, 6SN7 works very linear at 10mA. So he suggests me to replace 39K resistor for the plate to 10K and replace these 470uF caps at C1/C2 with a current source (i.e. LM334/LM134). I wonder if it would be a good move and looking for other plans to try as well.
I recently built a preamp and has some noise issue. It is not like hum, buzz, or hiss but sounds very much like white noise. Then I listened to some suggestions, reworked some parts and changed some chokes, filament supply, etc. None of them really worked for me, but reducing gain definitely helped. I tried adding a 1K resistor with a 100uF cap paralleled to the plate resistor for 6SN7 (39K at R7/R8). The white noise is completely gone. However, by doing so, the sound becomes quite weird. Sounds very analytic and the black magic of its midrange is totally gone. So I have been seeking for a way to reduce the gain and keep the original signature of the sound and might improve it if possible.
Here is a way that my friend suggested me to do, please see the attachment. We want to have about 8-10mA plate current and about 15dB gain so I would have sufficient gain and have 6SN7 to work at its sweet spot -- to my understanding, 6SN7 works very linear at 10mA. So he suggests me to replace 39K resistor for the plate to 10K and replace these 470uF caps at C1/C2 with a current source (i.e. LM334/LM134). I wonder if it would be a good move and looking for other plans to try as well.
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Noooooo someone is either jerking you around or you are misunderstanding their directions.
What ever the case may be, I can tell you that this circuit you posted is pretty much fubar.
I would just scrap that schematic and build a basic 6sn7 compound circuit. I would link a schematic, but I am having a hard time finding a good complete schematic for you to follow.
Maybe someone else can help me/you out here.
What ever the case may be, I can tell you that this circuit you posted is pretty much fubar.
I would just scrap that schematic and build a basic 6sn7 compound circuit. I would link a schematic, but I am having a hard time finding a good complete schematic for you to follow.
Maybe someone else can help me/you out here.
Questions:
1. What is the purpose of the circuit?
2. Who needs gain in 2020?
Proposal: if you don't have 50 year old equipment and/or if you don't listen to phono then you don't need any gain at all in 99% of cases*. Knowing tube people I can tell that there MUST be a tube in every device so if that itch is there I would advise to use a low gain circuit or a buffer circuit. It all depends what amplifier is connected to this device. For correct advice and for better understanding it would help to mention input impedance and gain of the power amplifier. If there is enough gain and let's say 50 kOhm or higher input impedance than your issue can easily be solved by omitting stuff *.
* It would not surprise me at all that a straight wire could be the best solution is this case. Greener, cheaper and no added noise or useless gain.
1. What is the purpose of the circuit?
2. Who needs gain in 2020?
Proposal: if you don't have 50 year old equipment and/or if you don't listen to phono then you don't need any gain at all in 99% of cases*. Knowing tube people I can tell that there MUST be a tube in every device so if that itch is there I would advise to use a low gain circuit or a buffer circuit. It all depends what amplifier is connected to this device. For correct advice and for better understanding it would help to mention input impedance and gain of the power amplifier. If there is enough gain and let's say 50 kOhm or higher input impedance than your issue can easily be solved by omitting stuff *.
* It would not surprise me at all that a straight wire could be the best solution is this case. Greener, cheaper and no added noise or useless gain.
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The "preamp" and the millions of designs and units it resulted in is indeed part of hifi history. It was useful for volume control and switching, neither of which require gain, and for low output like tape heads, microphones and vinyl. Sometimes but not always it included a step-down. As Jean-Paul says - what is its function in 2020? Line output is accepted as 2v and that should drive an amplifier to full output. If you only need digital, then the DAC pretty much replaces the preamp.
The impression I get is that a large part of the motivation of preamp users is to get a "good sound" - one they like. So why not just put your favourite preamp tube in the first stage of your amplifier? That way the gain is doing something useful. If you like a DHT, use one to replace your 12A*7 or whatever and then redesign the rest of the amp.
There's a lot of nostalgia in the classic preamps, but there's nostalgia in 78rpm records and for many of us, 33rpm LPs. I have nothing against the love of retro - it's human and supports a lot of valuable culture. But it has its place.
Well, vinyl is more common than some people might think and the typical phono preamp output is in the 200 mV range. With a 1-2 Vrms sensitivity power amp that kind of preamp must work.
That kind of circuit, regardless of personal taste, must actually work fine even if the power amp has 0.5 Vrms sensitivity and is used with CD player just changing the output to cathode follower. I say "must" because I still have these sort of things and work...no noise, unless I attach my ear to the speaker.
That kind of circuit, regardless of personal taste, must actually work fine even if the power amp has 0.5 Vrms sensitivity and is used with CD player just changing the output to cathode follower. I say "must" because I still have these sort of things and work...no noise, unless I attach my ear to the speaker.
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The second stage should be a cathode follower. That is the most effective gain reduction. And it could be directly coupled to the 6SN7. Also I cannot see the plate load of the ECC99.
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IMO issues are best solved at the root. So in the case of a 200 mV phono preamp one better upgrades this one to 2Veff output like any other source. This to avoid large differences in volume when switching sources and/or the terrible deed of attenuating 2Veff sources (which is a serious crime that must be punished). There is no more neutral preamp than a straight wire with 2Veff sources and a normal gain power amplifier. That is one likes HiFi, if one likes an effect box then it is another matter.
The straight wire will give lowest gain, highest SQ, blackest black, ultra low noise etc.
The straight wire will give lowest gain, highest SQ, blackest black, ultra low noise etc.
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Large? Typical CD players, playing disks, have 0.4-0.5 Vrms actual output on average despite it says 2V rms in their datasheet. That 2V is more like a max output.
In fact some (expensive) recent phono-preamps have 0.5V output probably for that reason but most of phono preamps I know are still rated as 200mV output. 0.2 or 0.5 cannot be a difference that turns a good preamp into a bad one.
The bad thing about the design above is just that 2 common cathode stages are excessive for a line preamp. Not even vintage stuff had that! They had a second stage if they used tone controls that typically caused 20 dB attenuation or if they used feedback. Even if the Ecc99 plate is attached to an ouput transformer with (typical) 4:1 stepdown ratio, gain is still too much.
In fact some (expensive) recent phono-preamps have 0.5V output probably for that reason but most of phono preamps I know are still rated as 200mV output. 0.2 or 0.5 cannot be a difference that turns a good preamp into a bad one.
The bad thing about the design above is just that 2 common cathode stages are excessive for a line preamp. Not even vintage stuff had that! They had a second stage if they used tone controls that typically caused 20 dB attenuation or if they used feedback. Even if the Ecc99 plate is attached to an ouput transformer with (typical) 4:1 stepdown ratio, gain is still too much.
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Passive control centers can't drive long interconnect cabling. "Unity gain", AKA buffered control centers dispose of that issue, when (as very frequently is the case) line stage is unnecessary. Remember, many power amplifiers are well into clipping, when fed 2 VRMS.
Many options are available, when line stage gain is truly needed. 2 examples I'm associated with are provided. I claim no more than solid performance and comparatively limited gain amounts.
Many options are available, when line stage gain is truly needed. 2 examples I'm associated with are provided. I claim no more than solid performance and comparatively limited gain amounts.
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Your measurements are off. In electronics we calculate worst case always. That is 0 dB signal level which unfortunately is reached quite often.
As stated the issue if often only with the phono preamp so that is the right candidate for an upgrade. A simpel OPA1656 gain stage behind a classic phono preamp is a very neutral and more efficient solution that solves it at minimal cost, no added noise etc. Or, if it really must be a tube, design a simple gain stage to make the phono preamp 40 years younger in specifications. When all sources are around the same signal level one big annoyance has been solved, the tedious difference in volume when switching sources. Even when the phono preamp would produce 0.5Veff there would still be a large difference in volume, please ask around why people build these devices and you'll hear the same. That is why somewhere in the eighties the 2Veff standard was created. Anyone that bought one of the first CD players quickly learned to turn down volume first when switching sources 🙂
True, that is why source section and volume control can be done in such a power amplifier that has enough gain and has high input impedance. Then a right value potentiometer or any other volume control can be right in front of the power amplifier. so no clipping except when one wants it to. So to speak an integrated amplifier. It saves quite some costs at no added expense in SQ, on the contrary. It also saves from wasting hours building an effect generator and many other people wasting time solving self created issues on DIY forums. It saves an extra casing, an extra set of interlinks and connectors etc etc.
It would help tremendously if DIYers first determine what the actual use of the circuit should/must be. For this one needs to know the won installation and what the signal levels and impedance equipment have. Many seem to just start off with a certain tube in their hands and make something with some purpose that has some use in some situation with some other device that some people should solve as it has some problems 😀 I am speaking from experience here. It would be more efficient to build stuff that enhances the chain instead of burning hours building a device around a random tube that adds nothing but issues. At least, this is what I see in practice.
I am very sure that in most cases (certainly with a tube power amplifier) the only things that are really needed are source selection and volume control. Less = more.
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Take ANY commercial integrated amplifier and read specifications. Max power rated for 200 mV input, max 300 mV. They work just perfect despite your personal and funny interpretation of electronics.....
Yes they work just perfect despite my personal and funny interpretation of electronics especially of the 2Veff specification/standard of sources 😀
If 200 or 300 mV is needed for full output building an extra high gain stage/preamp for a 200 mV output phono stage (let alone a CD player with 2Veff output) seems even more questionable, funny guy 🙂
If 200 or 300 mV is needed for full output building an extra high gain stage/preamp for a 200 mV output phono stage (let alone a CD player with 2Veff output) seems even more questionable, funny guy 🙂
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Are we really talking about a preamp or a line stage?
I have a SS amp that will make 300W RMS from 1V input. I also have a tube amp that requires ~6V of drive for full output. I turn the gain on the SS amp down to match the tubes.
My current preamp/line/headphone amp (there's a thread about it) will take 1V input and make 14V output with it. Normally this is excessive for a line stage, but there are some cases where the dynamic range of some classical music requires the full swing of the volume knob.
It makes no hum/hiss - even on headphones.
I have a SS amp that will make 300W RMS from 1V input. I also have a tube amp that requires ~6V of drive for full output. I turn the gain on the SS amp down to match the tubes.
My current preamp/line/headphone amp (there's a thread about it) will take 1V input and make 14V output with it. Normally this is excessive for a line stage, but there are some cases where the dynamic range of some classical music requires the full swing of the volume knob.
It makes no hum/hiss - even on headphones.
200mV senisivity is valid for CD player too as most integrated amplifiers do not have a phono stage. Don't go round in circles please...intergrated amps and preamps have a volume control....
Using the standard 200 mV sensitivity of preamps/integrated amps and typical power amp sensitivity going from 1 Vrms to 2 Vrms the gain should be from 14dB to 20 dB. With 6SN7+ cathode follower gain will be around 22 dB. Then there is something called volume control. These are the numbers. Not my my numbers....
What is wrong about the initial circuit posted is that there are 2 gain stages and gain is really excessive. That's all.
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Hi Mona, that is a good idea but what about the PSRR of the circuit? I guess it will need a quite good (regulated) symmetric so double expensive PSU. I think I would still use an output capacitor in your circuit though 🙂
I think it is time the OP gives some clarification what he exactly expects of the circuit and what the purpose is. Otherwise this thread will be a venue of opinions without a clear goal.
I think it is time the OP gives some clarification what he exactly expects of the circuit and what the purpose is. Otherwise this thread will be a venue of opinions without a clear goal.
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Thank you for your help sir. This circuit is supposed to be a classical /simple circuit to my limited knowledge. ECC99 drives a pair of output transformers may be a info so that this circuit does not seem to be fubar?
I use this preamp for driving longer cables and adding a little bit tube magic into the sound. Indeed, I can accept an unit preamp, I just need a little bit gain from 6SN7 to balance out the gain loss from its output transformers (8:1 if I remember correctly).
This circuit drives a pair of output transformers if it explains the plate load of the ecc99. I wonder how should I modify my circuit to make the second stage a cathode follower and if it can still fit the original design (driving a pair of output transformers and work with the original B+)? Thank you very much for your help sir.
Eli, what's the purpose of the LTP in the second circut? Is it there just to cut the gain in half?