Fender FRONTMAN 25R amp with LM3886 instead of TDA1514?

[jordileft]

Hi, I'm planning to build a Frontman 25R, but I can't find TDA1514 anywhere. I've been looking for a replacement for that chip and I've seen that LM3886 is a good replacement, but I'm just an amateur on electronics so I don't know if I have to change the schematic to make this replacement, could somebody help me? thanks!

[DJPhil]

You'll almost certain to have to make some changes and adjustments, as they don't have the same pinout (or even the same number of physical pins).

This is how I'd approach things . . .

Start out by printing up the respective datasheets. Take the Frontman schematic and figure out what the gain is set for, and why the surrounding components are there. Odds are pretty good that the power amp block is similar to the datasheet. Then you can translate that over to an equivalent circuit for the 3886.

That would do most of the legwork for the pros, and they'd have an easier time spotting trouble in a prototype circuit than having to design it from the ground up. I'll help if you get stuck, but I've got to get some rest at the moment so I don't have time to drive into the details right off.

Hope that helps some.

[jordileft]

Hope that helps some.

Thanks, it helps a little bit, but I don't have much confidence on my ability on achieving the right changes, 'cause it's my first SS amp. I'll try and I'll let you know!

[J M Fahey]

Hi Jordi.
Agreeing with the earlier suggestion and extending it a little:
I think the full Frontman is a bit too much to start, specially because an important part is unavailable.
Start by making a power amp and its power supply, either an LM3886 or TDA7294.That part of your amp stays forever.
Then you can make a very simple preamp , use it driving what you already have.
Later you can build more and more complex preamps, but going step by step.

[jordileft]

I think the full Frontman is a bit too much to start, specially because an important part is unavailable.

Despite being my first SS amp I actually have build a tube amp (a Fender Blues Junior similar amp) and some effects pedals, so I'm not a beginner on building amps, but my electronics knowledge on amps is just a little more than the basics. The problem for me is not building or the complexity of the preamp, the problem is having a part on the schematic that I can't find nowhere, so I think that if with your help we can find the changes needed the rest won't be difficult.

If someone wants to take a look on my tube amp you can see a video on this link:
http://www.youtube.com/watch?v=_rX-Og3EuXs

[DJPhil]

Despite being my first SS amp I actually have build a tube amp (a Fender Blues Junior similar amp) and some effects pedals, so I'm not a beginner on building amps, but my electronics knowledge on amps is just a little more than the basics. The problem for me is not building or the complexity of the preamp, the problem is having a part on the schematic that I can't find nowhere, so I think that if with your help we can find the changes needed the rest won't be difficult.

No worries. That's a sweet sounding tube amp!

Looking at the schematic, everything from R55 (which I think is limiting the current draw from the opamp buffer upstream) down is your power stage and should look somewhat familiar. Essentially you'd be replacing C33 (DC blocking cap) down with the LM3886 reference schematic and adjusting the feedback and gain to suit. The TDA datasheet is short and sweet, but the LM3886 has a heap of appnotes explaining the reference circuit and changes you can make to it. You may have to tinker a bit with the finished product to be completely satisfied, but that's half the fun!

I think you'll be surprised how easy it is. These sorts of chips are designed for low external parts count, so they tend to be fairly simple to use. You can also copy off of a schematic for a 3886 powered amp and see what they did. I've got no knowledge of amp models, so I can suggest any, but I'm sure there's folks around who'd probably be able to give you a half dozen examples.

I know that wasn't to specific, but I hope it helps some.

[jordileft]

No worries. That's a sweet sounding tube amp!

Looking at the schematic, everything from R55 (which I think is limiting the current draw from the opamp buffer upstream) down is your power stage and should look somewhat familiar. Essentially you'd be replacing C33 (DC blocking cap) down with the LM3886 reference schematic and adjusting the feedback and gain to suit. The TDA datasheet is short and sweet, but the LM3886 has a heap of appnotes explaining the reference circuit and changes you can make to it. You may have to tinker a bit with the finished product to be completely satisfied, but that's half the fun!

I think you'll be surprised how easy it is. These sorts of chips are designed for low external parts count, so they tend to be fairly simple to use. You can also copy off of a schematic for a 3886 powered amp and see what they did. I've got no knowledge of amp models, so I can suggest any, but I'm sure there's folks around who'd probably be able to give you a half dozen examples.

I know that wasn't to specific, but I hope it helps some.

Why should I change C33? this filter cap value has nothing to do with the amplifier used, just with the fc, isn't it? I've been looking the datasheets and schematic but I don't know how to calculate the gain of the TDA1514 on the Frontman schematic. Otherwise, I don't know what would be the difference from the LM3886 datasheet figure 2 feedback which has Rf1, Rf2 and Cf than the feedback on Frontman schematic, with Rf1, Rf2 and a resistor replacing Cf and a capacitor tied to ground between that resistor and Rf2. If I knew the gain on the frontman TDA1514 gain and how the difference on the feedback between LM3886 datasheet and Frontman schematic affects on the formula of gain which appears on LM3886 datasheet, I'm sure that the most difficult part would be solved. Could someone help me with those two things?

[jordileft]

I've been thinking and I hope someone could tell me if I'm right, no matter what's the gain on the frontman schematic with TDA1514, as gain is the relation between Vi and Vo given by the components on the feedback, then any components on the feedback on a TDA1514 would be the same gain than in LM3886 if the values are the same, is that right?

[DJPhil]

Why should I change C33? this filter cap value has nothing to do with the amplifier used, just with the fc, isn't it?

I said that to be conservative, but yes, it's essentially part of a 50Hz high pass filter. If I were to build a separate board for the power amp section I'd have put everything from C33 down on it, it's just a conceptual delimiter.

I've been looking the datasheets and schematic but I don't know how to calculate the gain of the TDA1514 on the Frontman schematic. Otherwise, I don't know what would be the difference from the LM3886 datasheet figure 2 feedback which has Rf1, Rf2 and Cf than the feedback on Frontman schematic, with Rf1, Rf2 and a resistor replacing Cf and a capacitor tied to ground between that resistor and Rf2. If I knew the gain on the frontman TDA1514 gain and how the difference on the feedback between LM3886 datasheet and Frontman schematic affects on the formula of gain which appears on LM3886 datasheet, I'm sure that the most difficult part would be solved. Could someone help me with those two things?

I can't say I blame you there. The datasheet has a note on the closed loop gain spec on page 7 for calculating gain. The feedback path is a bit convoluted, but the info in the LM3886 sheet regarding the feedback loop applies to the TDA1514. The gain works much like an opamp, with the extra bits rolling off gain at DC and high frequency for stability. The gain calculation for the TDA1514 is roughly equivalent to R3/R2 on the test circuit. Looking at the Frontman schematic and taking R58 in parallel with R59 + R60 I get a gain of about 13, compared to the datasheet circuit of about 30. That sounds . . . close to what makes sense.There may be something going on I don't understand, so I'll pass on certainty and refer it to the pros.

In the end, you can build the preamp section up to around C33, test the output, and configure the LM3886 for whatever gain suits you. There's a guide at the end of the datasheet to help. This is probably what I should have advised in the first place, but I didn't know the Frontman circuit would be such a beast!

I've been thinking and I hope someone could tell me if I'm right, no matter what's the gain on the frontman schematic with TDA1514, as gain is the relation between Vi and Vo given by the components on the feedback then any components on the feedback on a TDA1514 would be the same gain than in LM3886 if the values are the same, is that right?

Sort of, but frequency matters too. There are factors which might cause the specific values of the resistors to change, but the ratio of resistances between Vo and Vi, and Vi and ground will mostly determine closed loop gain. I wouldn't copy/paste the parts values, but it should be close.

Hope that helps.

[jordileft]

Sort of, but frequency matters too. There are factors which might cause the specific values of the resistors to change, but the ratio of resistances between Vo and Vi, and Vi and ground will mostly determine closed loop gain. I wouldn't copy/paste the parts values, but it should be close.

Hope that helps.

But resistors value its fixed between the tolerances, isn't it? are capacitive and inductive reactance which changes according the frequency. And being a negative feedback amplifier to calculate gain just needs to equal Vin+ and Vin-, I'm not going to make all the calculations, but it will be calculated in the same way on TDA1514 than on LM3886, I think I'm sure about that. I've attached a drawing of  how I would do this schematic section (keeping the previous and later sections).
One more question, what does D10 and D11 on the Frontman schematic do?
Thanks for your help!!

[J M Fahey]

Hi jordileft (Catalan perhaps?)
Nobody challenged your electronics knowledge, far from it, but if you ask, you will get an answer.
What I meant is that SS amps are more complex than tube ones (for a home builder), specially because you have to design a PCB which is a PITA, and easily 10 times more difficult than the actual amp construction.
Tube amps usually have less components, can be built on eyelet or turret board , which is very forgiving, any component can easily be replaced (either Mods or repairs) 1000 times, and most or all wiring is made with ..... actual wires, which to boot can be dressed at will, while a PCB is a "fixed/frozen" design, replacing a component can easily tear the corresponding pad, which is, after all, a thin foil of copper glued to the board.
As of the Mod/substitution.
Basically you need to calculate nothing: both power amp chips are big Op Amps.
Gain, frequency response, etc. are set by the external components, which can be the same.
Differences?
Pinout     (1514)  -  [3886] <-- attention to the bracket type
+in               (1)  -  [10]
-in                (9)  -  [9]
V+               (6)  -  [1,5]
V-                (4)  -  [4]
Out              (5)  -  [3]
Bootstrap     (7)  -  [n.c.]
Ground         (  -  [n.c.]
Mute         (3,2)  -  [8] <-- beware RC nets are connected in a different way, and in a third way on the Frontman; you must use the Datasheet 3886 version.
Recheck what I say, I'm switching back and forth between PDFs and may have made a mistake.
I repeat that you should build and debug first the power amp section, which is different from the original, and only after that tackle whatever preamp you feel confident to do.
When building something new, different , it pays to go step by step.
I myself started 42 years ago with a tube Gibson GA5 straight from Jack Darr's book (PTP on terminal strips), then a Blackface Bassman also on terminal strips, then **the preamp** from an Ampeg BT15 built on Veroboard (the PCB was too much for me) driving a Kit built 100W SS Motorola amplifier. See that the PCB was a big problem for a beginner.
42 years later I have built and sold over 10000 amps (no, it's not a typo, over ten thousand) and currently make my own PCBs, Cabinets, Chassis, Panels , Speakers, Transformers, and even knobs , strip handles, rubber feet and metal corner protectors, but it took *years* and lots of $$$ to be able to make all those things.
Good luck  :tu: :tu:

[jordileft]

Hi jordileft (Catalan perhaps?)
Nobody challenged your electronics knowledge, far from it, but if you ask, you will get an answer.
What I meant is that SS amps are more complex than tube ones (for a home builder), specially because you have to design a PCB which is a PITA, and easily 10 times more difficult than the actual amp construction.

Yes, Catalan!! you know Catalonia?
Don't worry, I wasn't offended.

beware RC nets are connected in a different way, and in a third way on the Frontman; you must use the Datasheet 3886 version.

What net do you mean?

The PCB thing, would not be a problem, I've already done more complex PCB on other projects. Otherwise I'm thinking about doing it all with soldering paths and bridges. I really enjoy doing that.

I repeat that you should build and debug first the power amp section, which is different from the original, and only after that tackle whatever preamp you feel confident to do.
When building something new, different , it pays to go step by step.

I think I'll build the whole schematic, 'cause there's no critical change, and checking other schematics that use LM3886 the only different thing is the feedback section, which is different 'cause the gain is different.

Thanks for the help!

[jordileft]

I have one more question, 'cause this part of the schematic really confuses me. The switch S1-A and S1-B, isn't it doing a loop? what's the function of S1-B? if there was no S1-B the function would be the same, isn't it? In that case I don't understand why it's not a one circuit switch. And why is S1-A 1 and 2 pins connected? Isn't those loops undesirable? can somebody help me?

[DJPhil]

Just looking at it I'd say that S1B is simply unused. It may be that there's more to the system on a different model that uses the same schematic, and in this version it's just grounded to keep it out of trouble. The loop is barely there in practice, as pins 3, 4, 5, and 6 will be shorted to ground on the circuit board.

[teemuk]

I'm pretty sure that Fender just found out they can buy a huge stock of DPDT switches on a cheaper price than buying also ample inventory of SPST and SPDT switches would have cost. The Double Pole Double Throw -switch can be configured to do the same things as all those other switches, but the other switches can't be made DPDT.

On an (sort of) off topic side note, this type of footswitching scheme is one the most cunning circuits I've ever seen in Fender amps, though it's not obvious from a simple amp model as this one with just a single footswitchable feature: the footswitch alters the amplitude of an AC signal, which is then rectifed and used as an input for a comparator circuit acting as a switching logic. The cunning part is, the comparator circuit can provide different switching functions like.
Vin = 0V: feature 1 on
Vin > 2V: feature 2 on
Vin > 4V: feature 3 on
and so on.
Furthemore, because this amplitude information is carried on as AC signal the comparator switching logic can work on interpreting both positive and negative voltages, therefore doubling the amount of switchable features. Furthermore, all this information can be carried in just two wires (AC and GND) to the footswitch. Furthermore, the same AC signal can power on LEDs within the footswitch.

It's absolutely brilliant.