I see, and it's an attractive idea, but won't this just slightly extend the time the amp will be operable?

I estimate the amp will draw 2.24 amps with a 20W 4 ohm speaker, giving about 2 hours use on battery only.  The charger I have is rated at 600mA 0.6A so I'm assuming it won't provide enough current to power the amp in operation or keep the battery topped up.

Am I right?

I want to build a 12V battery-powered amp employing a TDA2005 and would like some advice about the power supply.

I propose using a Ritar RT1250 12v 5.0Ah SLA battery to power the amp but I'd also like the option to run the amp from the mains using an AC-DC adapter.  I'd also like to be able to re-charge the battery without having to remove it from the amp cabinet.

I'm guessing it would be inadvisable to draw power from the adapter via the battery i.e. connect the battery in series with the adapter because the current would eventually damage the battery, so what would be the simplest way to achieve my goals?

Swell.  I'll pop a few pictures on in a few days time.
Nick.

At an auction yesterday, for just a few quid, I picked up an old practice amp (pic attached). It took some time to identify the maker but it turns out it was made in the 1980s by company called Deanvard, based in Peterborough, England.  The company made a small range of SS guitar amps from a 10W practice amp (which is what I bought yesterday) to a 100W Bass amp.  I don't think they had much of a reputation and the company closed in 1991.

What I am wondering is if anyone has any schematics for this company's amps?

If so, perhaps you would be good enough to post them on the site here, or perhaps let me have a copy.

Many thanks

Nick

Hi Roly

First off my apologies for not replying to your post sooner. It's been very hectic for a few days  :duh
Thank you very much for explaining the power calculations. That's most helpful and useful. The information is safely stored away

I have vague recollections of seeing the Black and White Minstrels on TV when I was a kid. My mum and dad loved them! Very different days those.

I'm going to build the amp as per the schematic and shall be ordering the components in the next few days. I'll keep you and JMF posted on progress.

Thanks again

Nick

Not at all scary Roly, quite the opposite in fact 

My comment about being almost too afraid to ask was due to my own feelings of inadequacy and embarrassment. All of you have been extremely accommodating and helpful and I am indebted to you.

I read Lauren's thread and it would seem she has either lost heart or simply run out of time. I noted she had a gift deadline. Either way it's a shame and I echo your hope she returns to fulfil the project.

I have a bit more knowledge than she, but not long ago I was in the same boat. Just 8 months back I had absolutely zero knowledge of electronics. I had no idea about Ohm's Law, what resistors, capacitors or any other components did, and no idea about audio whatsoever. I am quite surprised at what I have learned, but there are still so many gaps in my knowledge and I often feel I know nothing! My first love was 1920's valve radios and that's what got me interested in electronics. I read as much as I can, but I'm impatient to make things and see them working. The reward in doing so inspires ever more ambitious projects. Unlike many of this forum's members I do not play an instrument. It is my son who is the musician and for a 16 year-old he is quite accomplished. This amp project is for him. He likes playing and I like making things. A perfect combination  :tu:

Is that you in the picture? It gave my son a good giggle. Not in a disparaging way, just the difference in generations. As a heavy metal fan he looks as you might expect! The closest he gets to a suit is playing cards.

Can I ask a few more dumb questions?

• What is the advantage of your proposed speaker wiring over that currently in my circuit?
  
• If I adopt your speaker arrangement do I retain C12-C15 and R13-R13 as shown on my circuit (attached), or do I have to rewire pins 11, 2, 7, and 4 differently?
  
• Can I get away with a 15v 10W bulb, rather than 15W? (They are more readily available and cheaper).
  
• What wattage can I expect this final circuit to be? I had a go at trying to calculate it using Watts = Amps x Volts having first worked out amps using I=V/Rt but the result I came up with didn't make sense.
  

Thanks
Nick

Hi Roly

Thanks for the feedback. I will make the suggested alterations. I am almost too afraid to ask, but is this what you mean re the lamp?

Thanks
Nick

By asking for your indulgence one more time I of course meant in the context of your generous help and your patience too. As I mentioned at the outset of this thread, I am a novice when it comes to solid state circuitry and to amplifiers too so I really do appreciate the willingness with which you freely share your knowledge and time.

I have noted all the points each of you have provided and (somewhat surprisingly) by and large I get it too! For that, my thanks  :tu: Now I think there remain just a few points for which I could do with a bit more clarity if that's OK?

(1)   - JMF -   "your first one was a conventional 1/8" plug/jack (not recommended for power supplies) and it was wired inverted."
Doh,  :loco What can I say?!   I told you solid state circuitry was new to me. They didn't use jack plugs like that in the 1920s. Those old valve radios were so much simpler! At least I now know the difference and to be more careful about which symbol to use 

(7)   - JMF -   "original is single 741 op amp. RC4558 is fine, but is double. You have to use a single there (TL071 is fine) or neutralize unused Op Amp"  ...also...  "Beavis audio is not 100% trusty, he's leaving an open loop, floatin Op Amp in the same package and die as the used one. That's unstable, will probably oscillate or latch up or do any amount of trouble. The almost original MXR schematic is this one:"
Ah, that original MXR schematic is quite a bit different! So it begs the question, should I swap over, or is my RC4558 one with your corrections now problem free and suitable for my son's (heavy metal) needs?

(1, 2 & 4)   - ROLY -   "there is no current limitation into the battery - this is vital or the battery might cook/boil depending on how grunty your supply is, or cook the supply if the battery is flat....and...To act as a charge limiter the lamp should go between the incoming supply and the voltage sense point, D2."
Understood. Have I got it right now?

(6)   - ROLY -   "Stick the 1nF in across the guitar if you wish, but I think you will be tweeking it when it sounds muffled and lacking in tops."
Can I just check you are referring to C3 and that you agree with JMF's variation on this latest schematic?

Oh before I forget, JMF you mentioned "you may add a spdt switch which both lifts diodes out of circuit and adds a fixed, say, 10k resistor in parallel with my suggested gain pot so Op Amp becomes a very clean 3X gain preamp" If and when you can find time I would be most pleased to have your drawing showing how.

Obviously I can't repay any of you for your kindness in like, but once we have the schematic finalised what I will do is build the amp and having done so I will then post an instructable on this site acknowledging your contributions so that others can replicate it for themselves and benefit from your expertise. I hope that meets with your approval.

Thanks again.

Nick

Attachments:
Revised schematic v03.
Oops! Just noted I have annotated C3 as 220nF. It should read 220pF.

Thank you each of you for the guidance, it is really appreciated.

OK. I have numbered all the components, which I should have done from the outset, so my apologies there and I have attached an updated schematic. In the process I tried to address each of the points made by 'g1', 'Roly' and 'JM Fahey' but I confess I haven't succeeded entirely so if I could ask your indulgence again to correct any omissions and mistakes I would be most grateful. I have followed the numbering convention in JMF's post for continuity.

(0)    "the volume pot wiper is drawn going straight to ground"
I have checked a number of schematics employing the TDA2005 and each show a path to ground on the input line. I have amended the schematic to show a direct path from the volume pot (VR2) to pin 1 of the TDA2005 via capacitor C9, but should I retain a path to ground too, as highlighted on the schematic?

(1)    "the power in jack is upside down"
I have checked the schematic I was using for this part of the circuit and I am pretty sure I have connected the power in jack as was shown. It came from this YouTube video (https://www.youtube.com/watch?v=c_XkuKeSLGs) and I have attached a picture of the circuit too. Am I being dumb?

(2)    "you will need to limit battery loading current; simplest is adding in series a plain vanilla 12V 15W`car lamp"
OK, I think I have done that correctly. See L1 as highlighted on schematic.

(3)    "To make operation point more defined, add, say, 4k7 to ground from node 1N4001/4k7 at 2N4400 base"
OK, I hope I have done that correctly. See R19 as highlighted on schematic.

(4)    "put the fuse in series with the battery (and on the +ve end 'coz I'm conventional)"
I have added a fuse as I believe has been suggested. See F2 as highlighted on the schematic. The original idea was to have a fuse in the charging circuit to protect the battery in case of adaptor and/or circuit failure. Not sure if that was or was not required or done correctly. In one or both cases I was thinking of a 1amp fuse, but can I get away with less?

(5)    "The input jack could be a shorting type"
Thanks for the suggestion, I may well switch to one when I order the components for the build.

(6)    "The 1nF right across the input will have a serious dampening effect on a passive guitar"
I am assuming you are referring to the capacitor now labelled C3 on the schematic attached. Is that right? As JMF says, it is the MXR original design. Are you recommending changing it (C3) to 220pF instead?

(7)    "original is single 741 op amp. RC4558 is fine, but is double. You have to use a single there (TL071 is fine) or neutralize unused Op Amp"
I have used the attached schematic for the MXR+ which recommends the RC4558 but it didn't mention neutralising the unused Op-Amp. It would be most helpful if you could describe how to do so now I have labelled the components. Thanks.

(    "I've successfully modded the circuit to use a much easier to find Audio pot by shorting gain pot shown (so that leg of NFB is just 4k7 in series with 4n7) and replace 1M feedback resistor with a 1M audio pot"
Sorry, I don't understand

(9)    "1nF in series with 10K means 16KHz , quite acceptable"
So no change to circuit made.

(10)    "connect the headphone jack with both hot contacts tied together (L+R) to one TDA2005 out, adding in series a 100uF cap to stop DC (there's +6VDC present) plus 100 to 470 ohms (your choice)...and... The other headphone out should be grounded and nothing else. As shown you are shorting, both DC and AC, the lower TDA2005 vpower amp."
I confess the connection of the speaker(s) to the dual outputs of the TDA2005 completely confused me. The bridged application schematic (attached) shows a single speaker of which one terminal is connected to pin 10 (output 1) and the other terminal to pin 8 (output 2), whereas I would have expected either pin 10 or pin 8 to go to the positive terminal of the speaker and negative straight to earth (-). I'm afraid I still don't understand how to wire this correctly for two speakers plus optional (switched) headphones in spite of your explanations! Sorry.   

(11)    "either use 2 8 ohm speakers, or just a single 4 ohms one, or my favourite weapon: I use TWO TDA2005 , each driving its own (ejem, FAHEY brand ) 4 ohms speaker"
In light of item (10) above I'm undecided how to proceed (2x8ohm, 1 larger 4ohm, or 2xTDA2005 and 2x4ohm).

(12)    "you would need to design and make a PCB for this amp, but for a single one (or a couple) you can very well build it on perfboard"
Yep, as it's a one-off I plan to do it on perfboard or prototyping board.

(13)    "you may add a spdt switch which both lifts diodes out of circuit and adds a fixed, say, 10k resistor in parallel with my suggested gain pot so Op Amp becomes a very clean 3X gain preamp"
Sorry, call me thick but I don't understand.

Attachments:
1. Revised schematic
2. Circuit I adopted for the MXR+
3. Circuit for TDA2005 for bridged application

Thanks

Nick

Thanks for the compliments regarding the enclosure design, but I can't take all the credit for it. I came across the case being used for something else and I thought it could be nicely adapted for use as a portable amp. As far as stereo is concerned, at the time I did the first post of this thread I did have a vague idea of designing an amp that could also double as a stereo amp for an alternative audio source (iPod etc.), but I abandoned that idea pretty quickly. No, it's going to be a mono amp with dual speakers for guitar only.

With this in mind I have adopted JMF's suggestion of the MXR+ and TDA2005 and have drawn the attached schematic. However, I do have this nagging feeling I have got something wrong, but for looking I can't put my finger on it. Perhaps someone could enlighten me? I have also added a circuit for the purpose of charging the battery from a 12v wal-wart with automatic cut-off. Once I get the circuit sorted out I can then get building .

Thanks

Nick

Ah, now that makes a lot of sense. I have found the relevant circuits and it seems pretty straightforward too.
Thanks JM, much appreciated.
Nick

Excellent advice JM, thank you

I will give serious consideration to your suggestion of dividing the cabinet into two and dedicating just one side to the speakers. My normal practice when designing, be it a homebrew valve radio or other project, is to first mock-up the unit from an old thick cardboard box. It's usually rigid enough to support the weight of the internals and all the design impediments can be overcome before the real build begins.

I have seen the quality of your work JM, and I know you make and sell professionally. At the risk of being too cheeky I note you say you have built many amps similar to what I need. I don't suppose you have a schematic you could share with me do you? Although I understood everything you proposed I have insufficient knowledge of solid state circuitry and guitar amps to design one myself from scratch.

Thanks

Nick 

As the topic title says, is there anyone out there who can help me design a circuit and schematic for a 10W+ stereo portable mini amp?

I am new to solid state circuitry, but I have a good working knowledge of vintage valve radios, so I can follow a schematic, assemble a unit etc. etc. I have just finished making my son a 'toy' 0.5W 'baccy-tin' amp and now want to make him a decent portable amp, still battery powered though (12v 4.5Ah sealed lead-acid), with optional DC adapter also used to charge the battery. This will be for practice play and for taking to the park/camping etc. He could easily buy himself a commercial unit, but I like making things and want to have a go as a surprise gift. Oh, I should mention he's a pretty good player and into heavy metal in a big way.

I'm not aiming to make anything fancy, no special effects required, just one 6.5mm input, volume, tone and gain controls, with the option to listen via the speakers or switch to headphone socket. For the speakers I was planning on two Jensen MOD5-30's. I have designed the cabinet which is 7" cuboid and the sides open out to create two wings, each of which will hold a speaker. I have attached a rough CAD drawing to give an idea and an image to show what it could look like when closed (I would use a different leather so it doesn't look so old-man!).

So, is there anyone who can help me with a circuit?

Thanks

Nick