Help required with some power supply & reverb circuit queries?

Discussion in 'General Electronics Chat' started by Ed Bray, Feb 1, 2017.

  1. Ed Bray

    Thread Starter Member

    Feb 1, 2017
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    I am having issues with a project I am making. The project is to use a couple of circuits from a Fender Blues Junior valve amplifier to make a separate stand alone Reverb Unit. I already had a Reverb tank with an 800 Ohm input and a 2575 Ohm output and the Blues Junior uses a Solid State circuit to both power, drive and recover the unit (it also uses the same reverb tank I have).

    Power supply circuit:

    BJ Reverb Power Supply.jpg


    I have a 240V - 20V transformer, this will supply the circuit with power, I have made both the circuits shown but have some questions before I actually fire it up.

    What is the purpose of C- and where does it fit in the grand scheme of things?

    The ground shown at the base of the circuit is also one of the AC incomers, is this usual and, will it be the same ground as required for the reverb circuit below?



    Reverb Tank Drive & Return Circuit

    BJ Reverb Drive & Return.jpg

    At the moment the only reverb control is R51 (50k pot), if I wanted to add a wet/dry pot and an input signal pot can anyone offer any ideas as to where they might go please?

    I have built both circuits on the same bit of breadboard type PCB, would I have to cut the link of the ground between the two circuits?

    Hoping you can help.

    TIA
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You can eliminate the components used to create C-, R37, R31 and C14.

    In the grand scheme of things, that is a poor circuit to use for audio work.
    The +15V and -15V supplies are half-wave rectified and will produce AC hum.

    You would be better to use a centre-tap transformer and use a full-wave bridge rectifier (or four single rectifier diodes).
    Use a 7815 for the +15V supply and a 7915 for the -15V supply. Watch the pinout of the 7915 carefully.
    You do not need the two 1N4004 diodes shown in the diagram below.

    [​IMG]

    Here is a similar circuit schematic:

    [​IMG]

    Replace 7812 with 7815 and 7912 with 7915 if desired.
    Select a power transformer that will result in about 3-5V excess voltage above 15V at the input of the regulators, i.e. you want about 18-20VDC with no load.
     
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  3. Ed Bray

    Thread Starter Member

    Feb 1, 2017
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    Thanks for the suggestion, however I am surprised that you think the circuits I shown will be noisy, these circuits are part of the Fender Blues Junior Amplifier Circuit and that is not known for being a noisy amplifier.

    Centre tapped transformers are not particularly abundant or inexpensive in the UK, can you suggest another circuit that would produce a similar output that is not centre tapped?
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    You could try using a bridge rectifier and aim for an output of 30V.
    Then create a voltage divider with two 1kΩ resistors to create a pseudo 0V reference between the supply rails.
     
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  5. dendad

    Well-Known Member

    Feb 20, 2016
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    The top power supply will be ok for low current as there is plenty of filtering, as long as the zeners stay conducting. And yes, you need the ground connection between the 2 circuits as it is the "0V" line. Don't cut the link between the circuits as it is needed. The power supply is +15V, 0V, -15V and if you cut the link, you will get only a 30V supply with no common reference.
    The "C-" looks like a -10V (aprox) that is used by some other part of the amp. You don't need to worry about that part of the circuit.
    An input pot can be connected with the wiper to the top of C30, the low terminal to the ground, and the high terminal to your input signal.
    Sorry, I don't know what a wet and dry pot is. I like to keep all my pots dry ;)
     
    Last edited: Feb 1, 2017
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  6. Ed Bray

    Thread Starter Member

    Feb 1, 2017
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    Thank you very much, that makes a lot of sense and does appertain to what I have already built.

    The wet/dry reference is an audio term for the mix with the dry signal being unaltered and the wet signal being fully altered, many effects units have an input signal control, a mix control and an amount of effect control in this case reverb. On the Fender Blues Junior amplifier the reverb effect is only controlled by a single control, I just wondered where I would put the other ones if I wanted them. :)

    I have just had a look at the transformer I have and it has 2x 0-20 volt secondarys, when AC voltmeter is connected across either of the 0-20 lugs I get 23 AC volts, if I put the voltmeter across the 2 outside lugs (with the centre pair of lugs not connected) I get 36 AC volts but with the centre pair of lugs shorted and the voltmeter across the outer pair of lugs I get 46 AC volts. Would this then be a 46-0-46 centre tapped transformer if I took a ground from the shorted centre lugs?

    If that is the case, is there an easy way of reducing the voltage down from the 46-0-46 volts to the 15-0-15 volts I would need?

    Thanks again for your help so far.
     
  7. #12

    Expert

    Nov 30, 2010
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    The (2) 20 volt windings would make a 20-0-20 volt transformer. It's running a little high on voltage because there is no load connected to it. It will load down to 20 volts if you use up the rated current for the transformer.

    I calculate the ripple voltage at 0.4694 volts peak to peak. That's the noise MrChips was concerned about.
    If the power line hum is any sort of a problem the 7815 and 7915 chips are way superior to a zener diode regulator.
     
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  8. Ed Bray

    Thread Starter Member

    Feb 1, 2017
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    Brilliant, that's great, thank you.

    I do actually have some 7815 and 7819 chips just wasn't sure about using them as I never had before. I'll have a look at one of those circuits above to power the TL072, I have most of the other components required.

    Thanks again.
     
  9. #12

    Expert

    Nov 30, 2010
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    Maybe...maybe not. 1/2 volt of ripple before the zener diode and considerably less after the zener diode and it's a judgement call. If you can't hear any power line hum, leave it alone.
     
  10. Ed Bray

    Thread Starter Member

    Feb 1, 2017
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    Ah well, it didn't take too long before I am back wanting more help.

    I build the original circuit and although the reverb worked it wasn't as clean a sound as I was hoping for, possibly because it was a stand alone rather than built into the Blues Junior so I decided to order a couple of additional caps and build one of the power supplies above using the 7815 and 7819 chips.

    I bought some 1000uf 35V Electrolytics and used 10uf 25V Electrolytics on the output side of the chips, I built the circuit on a separate board (glad I did) and fired it up, imagine my surprise when C1 decided to blow up covering me in a fine powder, as I had bought a pack of 5 I took a couple of readings (circa 30V) and decided that maybe it had been a dodgy Cap and fitted another, it wasn't very long before I found out that it wasn't a dodgy Cap when the second one also decided to blow. So I decided to revise my circuit and beef up the Capacitors. I also decided to build it on a breadboard so I could easily make any changes I needed to.

    As I didn't have any higher voltage 1000uf Caps I exchanged C1 and C2 for 2x pairs (in parallel) of 470uf 50V. I then beefed up the output Caps too to 10uf 50V ones.

    I then took a number of readings from the various places marked on the diagram below and checked them on the scope too. For some reason the 7915 chip does not seem to handle the higher voltage as well as the 7815 does and I can not get it anywhere near the -15 volts I am looking for. I did think that I may have blown the chip when the Caps blew (even though it was on the other side of the circuit from C1) so swapped it out, when that also had similar voltage readings I swapped it out again and when that was also the same I decided to phone a friend (or in this case post a message asking once more for some help).

    I have adapted the circuit diagram from above to detail the actual circuit I built along withal the readings taken at various points.

    If possible I would like to use the transformer I have and make the circuit give me the +15 Ground -15 volts that I am looking for so solutions that can do this would be greatly appreciated.

    TIA, Ed.

    7815  7915.jpg
     
  11. dendad

    Well-Known Member

    Feb 20, 2016
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    Did you wire the regulators up correctly?
    The 78xx and 79xx have different pinouts. The tab on the 78xx can be mounted to the case if it is 0V but the 79xx CAN NOT!!!!
    regulador-15v-7915.jpeg
    The 7915 has a max i/p volts of 35V.

    A careful look at your construction at your construction to check the caps are all in the correct way too.
    The caps need to be 35V rated at least, not 25V.

    Really, the original zener regulated supply would have been ok.
     
  12. Ed Bray

    Thread Starter Member

    Feb 1, 2017
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    Thanks, yes it was all wired correctly and the caps were installed correctly I did initially wire C2 and C4 incorrectly but caught it when I checked before firing it up, and I did originally use 35V caps which were the ones that blew, I then took them up to 50V. Neither tab is mounted to anything, I took voltage readings of both (to ground) and the 7815 tab was 0V and the 7915 tab had the input voltage of -31.79V on it.

    At this point, I am more concerned about why there is a discrepancy of the voltage regulation of the 7915 compared to that of the 7815, I need to know if there is something I need to be aware of in the future so would like to understand why this maybe acting like it is. I have checked the circuit many times, especially after having the Caps blow and can not see anything that is wrong. Since the Caps have been uprated to 50V I have not had any issues whilst it has been fired up. Having made it on a breadboard allows me to check and adjust as required and there are only 12 components (4x IN4007, 4x Caps for C1 & C2, 7815, 7915 and 2x Caps for C3 & C4), 6 on one half of the circuit and 6 on the other.

    Looking at the info you have supplied above it suggests a value of 22uf for C1 and for C0 10uf (which I am already using), I shall try an additional cap before the 7915 of 22uf, but I have to confess this has got me beat. I will try some different values for the various components and see if that makes any difference.
     
  13. dendad

    Well-Known Member

    Feb 20, 2016
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    Those caps are small, best to use tantalum types, as close as possible to the regulator pins.
    You still use the bigger caps as the bulk reservoir filter caps.
    With your 23VAC you would get up to 32VDC so 50V caps are a good idea. The o/p ones could still be 25V as the max should be 15V.
    The 3 terminal regs are a bit overkill as I think the original circuit could probably supply a fraction over 15mA max. before dropping out of regulation. Now you have something that can supply an amp per side, if the heat sinks are up it.
     
  14. Ed Bray

    Thread Starter Member

    Feb 1, 2017
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    Thanks for the suggestion. I have upped C3 and C4 to 470uf 50V (didn't have them in a lower voltage).

    I have managed to get the power supply supplying a constant +14.88V and -15.06V but have used a couple of resistors to drop the input voltage to the 7915. I have used a 750 ohm 3 watt resistor between the negative of C2 and the input of the 7915 and 1k5 ohm resistor (2x 750 ohm 3 watt in series) between the input of the 7915 and ground. I have checked the outputs on the scope and there does not seem to be any ripple.

    I suspect this may be a bit of a fudge and used the Voltage Divider Calculator at http://www.ohmslawcalculator.com/voltage-divider-calculator I put input as 24V, R1 as 750 ohm (only 3 watt resistors I had to hand) and 16V output and fortunately R2 came out at 1500 ohms, happy days.

    It has been running now for an hour or so and seems very stable and the 750 ohm resisters are only getting a little hotter than ambient temperature.

    Would still welcome any further suggestions as to other methods to achieve a similar result.
     
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