• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Single ended stereo tube amp subwoofer

Hazmoment

Member
2016-03-29 9:08 am
So Im building a chinese single ended stereo amp kit its around a 4.5Watts design.

Ive built this kit previously and Its pretty good. Infact I like it so much that this is my third one. The only thing I have noticed is that the transformers generate a lot of heat. The cores seem undersized in my opinion.


Anyway, This time Im looking at modifying it a little and have installed an RCA output on the back of the chassis which I want to use to connect to my powered subwoofer using the RCA input on my powered Sub.

My question is how to correctly wire this up to the amp:
Am i right to assume that because the Sub has a built in crossover it is ok to feed it straight from the grid of the first pre amp tube without needing any high frequency roll off circuit. And do I need to be concerned about effecting the impedance to the first stage tube if it is looking at the Sub input circuitry too? I guess I will just need to connect the sub to one channel only, but could balance between them if its more desirable. Any info you guys can provide would be appreciated!
 

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Ideally you would put in a cossover of some sort before the input to the amplifier to filter out the frequencies that will be handled by the subwoofer, this could be simple as a couple of cascaded high pass filters set around the desired frequency.

A model and schematic for your amplifier will help greatly, as well as for the subwoofer you intend to run.
 

Hazmoment

Member
2016-03-29 9:08 am
Sorry, heres a copy of the schematic. its not a branded thing, uses the 6n8p dual triodes and 6p3p’s for the output.

Thanks for the suggestion of an internal crossover lingwendel. what is the disadvantage of feeding the entire audio signal through to the Sub? Would the Subs internal crossover not just take the frequencies it needs anyway?
Happy to trial anything but Im no good at designing crossovers. Any thoughts on
The best source of info on designing a suitable crossover for this?
 

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pcan

Member
Paid Member
2015-12-31 4:57 pm
Most powered subwoofers built in the recent years have built-in low pass filters; that was not the case on older ones. The input impedence of the RCA input may be specified in the subwoofer manual. 10k is the most common value. This is one order of magnitude lower than the input impedence of common tube stages: a schematic of your amp is needed to do a proper check. Why don't you use the speaker level input of your sub instead?
I see that you connected the negative side of the filter capacitors to the ground bus with yellow/green wire, and the input power socket yellow/green wire is not connected to a dedicated bolt on the chassis. This is a violation of the current CE safety regulation (IEC 60065:2014).
 

Hazmoment

Member
2016-03-29 9:08 am
Hey pcan Should I be using black for the negatives? The dedicated earth to the chassis I havnt done but isnt forgotten. Im still busy wiring it up. Havnt soldered the earth terminal yet for this reason. I was just gonna use one of the nuts and bolts on the power trans.
Looks like 470k is the input impedance on my amp (schematic included) Can a small audio transformer be used? Can i test my sub input impendance simply with a multimeter?
Sorry but not keen on using speaker level idea, really keen on rca. Any other ways if a small trnsformer cant be used?
 

pcan

Member
Paid Member
2015-12-31 4:57 pm
I use black wire or bare wire on my builds for the negatives. EN60065 says: "Protective earth conductors may be bare or insulated. If insulated, the insulation shall be green/yellow except in the following cases: ..." "Wires identified by the color combination green/yellow shall be used only for protective earth connections". I try to follow at least this rule because green/yellow does have a very specific meaning to anyone trained on electrical works, and it looks ugly when used improperly. I believe that the transformer bolt is good enough on this context - the amplifier will never be able to pass a CE or UL qualification anyway, because the power transformer does not bear the required safety marks and is indeed built the way they were in the '60. Just be sure to scrape any paint near the bolt, to expose the bare metal chassis. The maximun resistence between the ground terminal on the input power socket and the chassis shall be less than 0.1 ohms - be sure to test it.
If you don't like to connect the subwoofer to the speaker terminals, connect it in parallel to your amplifier input - just install a RCA socket in parallel to the existing one or use a Y cable. Any modern source will happily drive togheter your 470K amplifier and the subwoofer. You will also hear less noise due to the lowered input impedence. Ymmv, but a try is very easy.
 
An idea to take signal off the amp, may be to disconnect the 220µF cathode bypass at the final pentode, add a 10Ω between negative of this cap and ground, and take signal from its cathode, as a cathode follower. This will subtract some gain from the amp because of degeneration at the cathode, but at very low impedance to attack a sub filter and post amp.
 

Hazmoment

Member
2016-03-29 9:08 am
Thanks Pcan. I would like to track down a copy of the Aus/Nz regulations regarding amplifiers. I looked into buying a copy but it was out of my price range. Would anybody have a copy on here?
I have considered your suggestion with using a Y connection on the input to the amp but I wont get any volume control via the sub. Id like it to come after the volume control on my valve amp.
Which safety marks are necessary on the transformer out of interest? I have no doubt that these transformers dont meet current safety regs.
Is there an IC i can use that will provide a LFE output by rolling off the higher frequncies? Im still keen on finding a solution to doing this. thanks for the info on colours I will replace my green & yellow wire for black where it is not a chassis earth specifically.
 

pcan

Member
Paid Member
2015-12-31 4:57 pm
I still cannot understand why safety standards aren't free to download. It is useful knowledge, often gathered from real events, that may prevent accidents and the associated social and human costs. As if the regulatory bodies didn't get enough funding already. But I will stop the ranting here. Australia is linked to the IEC, therefore the relevant norm should be AS/NZS 60065:2018 but I am not an expert and I may be wrong. Usually registered professional engineers, universities and schools have access to discounted subscriptions, or you may try to get a copy of the previous expired standard (it's almost identical, for your purposes).
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
....I see that you connected the negative side of the filter capacitors to the ground bus with yellow/green wire, and the input power socket yellow/green wire is not connected to a dedicated bolt on the chassis. This is a violation of the current CE safety ....

I'm not *sure* it is wrong to use the green/yellow as desired *inside* the chassis.

However the green/yellow in the wall cord MUST go to chassis as quick and solid as possible. This saves your life.

Here are OLD recommended color codes inside radio/TV in the US. Many English-speaking areas used to echo some of these colors. There are many standards. It does help debugging to be consistent.

Start with the "chassis" colors. TV color codes want stripe wire which is less available and probably overkill for one-off DIY. I omit a couple pages on Military color codes. It may be critical to find the "right" wire instantly while the bombs are falling, but we are not in that situation.
 

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I built a variable LPF for my sub - 120hz to 50hz I think, using a TL074. It's powered by it's own +15v 0v -15v power supply - little 12 0 12v tfmr, two 1000u caps and a 7815/7915. Two opamps are buffers, meaning you could take the signals off your volume pot wipers, especially if you use a 10M resistor on the IP to the buffers. The rest was a dual pot, a few resistors and caps, I designed it to use common value's, so no need for 3.156n caps or 2387r resistors.

There is loads of info out there on building filters - Elliott Sound Products - The Audio Pages (Main Index) as well as various free software filter design tools.

I'll see if I can dig the schematic out if it's any use to you. Andy.
 

Hazmoment

Member
2016-03-29 9:08 am

Hazmoment

Member
2016-03-29 9:08 am
I built a variable LPF for my sub - 120hz to 50hz I think, using a TL074. It's powered by it's own +15v 0v -15v power supply - little 12 0 12v tfmr, two 1000u caps and a 7815/7915. Two opamps are buffers, meaning you could take the signals off your volume pot wipers, especially if you use a 10M resistor on the IP to the buffers. The rest was a dual pot, a few resistors and caps, I designed it to use common value's, so no need for 3.156n caps or 2387r resistors.

There is loads of info out there on building filters - Elliott Sound Products - The Audio Pages (Main Index) as well as various free software filter design tools.

I'll see if I can dig the schematic out if it's any use to you. Andy.


Thanks andy. The Sub Im intending to use and most subs ive dealt with seem to have integral LPF’s built in. Do most solid state amps which have sub outputs on them filter the highs off internally even though subs all seem to have LPF built in? is this necessary for me to filter the highs off in my amp before going to the sub? Im sure I can send a full range signal straight to sub. after my volume control as long as the low impedance of the sub is not going to drag down the impedance of the amp. Keen to try the cathode connected Idea mentioned earlier. If its going to pull the gain down slightly in one channel i wonder what i can do to balance it.
 

Hazmoment

Member
2016-03-29 9:08 am
I'm not *sure* it is wrong to use the green/yellow as desired *inside* the chassis.

However the green/yellow in the wall cord MUST go to chassis as quick and solid as possible. This saves your life.

Here are OLD recommended color codes inside radio/TV in the US. Many English-speaking areas used to echo some of these colors. There are many standards. It does help debugging to be consistent.

Start with the "chassis" colors. TV color codes want stripe wire which is less available and probably overkill for one-off DIY. I omit a couple pages on Military color codes. It may be critical to find the "right" wire instantly while the bombs are falling, but we are not in that situation.

Thanks for this. I used green/yellow on the cap negatives only because its at the same potential as the chassis. I agree though that i shouldve used another color for this like black. chassis earth is still to be done in the pics included as is all of the audio signal wiring which going off the schematics for this kit kndicated a chassis connection for the input grounds too.
 
Is there an IC i can use that will provide a LFE output by rolling off the higher frequncies? Im still keen on finding a solution to doing this. thanks for the info on colours I will replace my green & yellow wire for black where it is not a chassis earth specifically.



I'm not sure about an individual IC but there is an eBay seller called KMTech who sells a Linkwitz-Riley 2-channel active crossover filter which does what you want, works well and comes with decent instructions (including calculations on varying component values to set the xover frequency). I've used it a couple of times on different projects and would recommend it if you have room for the extra PCB inside your project.
 

Hazmoment

Member
2016-03-29 9:08 am
Do I got the little amp finished up this weekend. I changed the green/yellow wiring for Black, grounded the chassis and wired it up more or less how I have done in the past. when commissioning it I had a horrendous 50hz buzz. I tried a few things and found the way to eliminate it was to take my RCA input chassis ground off and ground them direct onto the busbar. I ended up connecting my sub output to the second stage grid on R channel- I was expecting to hear a difference from my speakers between the L&R But couldnt hear any change. Any thoughts or Ideas on this? Cheers