Show posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

Show posts Menu

Messages - n9voc

Wow!  Eleven years since I last revisited this topic!


The inputs are AC coupled together, but not DC coupled.  This protects the IC from damage. in the eleven years since last posting - neither or nor the current owner of the amplifier has had any issues with this arrangement.

If you are speaking of the RCA and 1/4 inch jack, Those are isolated from the IC by the FET.  Again, no problems known. :dbtu:

Regarding the 220 uF vs 470 uf in the feedback circuit - the Xc of 470uF is about 4.2 ohms at 80 Hz, and Xc of 220 uF is about 9 ohms at 80 Hz. 

Looking at the feedback resistive setup - i think that going with a 220 uF would reduce the bass amplification response of the amplifier - by how much?  I have not a clue. :cheesy:
Schematics and Layouts / Re: Vox pathfinder stripped
April 13, 2012, 07:42:09 PM

You could try the lower value coupling caps, but that affects the frequency response of the amplifier (lower caps mean bass is cut).  I'm not saying it won't WORK, it just won't sound the same with less "oomph" in the low end than the original.

Amp Stages:
Again, the proposed removal will probably WORK, however, it will change just how the preamp responds in frequency by eliminating a stage.I'm not saying it won't WORK, it just probably won't sound the same as the original.

All that being said - it is YOUR amp, and you can build it the way you want.

Good luck!
Schematics and Layouts / Re: Vox pathfinder stripped
April 13, 2012, 07:50:14 AM
Good Eye!

A) Indeed, you can easily leave R6, R7, and C8 out without making a noticable effect on the signal.

B) The 1 K resistors:  I assume that instead of R13, (in the boost ckt) you mean R31 in the main preamp signal path, as well as R15 at output of second stage instead of R14,  (13 &14 in the boost circuit help with xsistor bias, I believe)
R15 provides a load to prevent the 2nd stage op amp from being directly shorted to ground when the two LEDs are conducting (during overdrive conditions)
R19 is a buffer (current limiter) for the optocoupler.  If eliminating the optocoupler, one can eliminate R19 and C18 as well.
I would say that R31 is a combination input buffer and oscillation preventer for the third stage (in the same way that a "grid stop" resistor is used in MANY tube circuits).
R35 serves as a buffer so that if you have the volume at full, and inadvertantly short the output of the volume pot - the op amp is not directly shorted to ground.
In short, yes - these are necessary.  The coupling caps serve as DC isolation between circuit sections.

C) The code on the capacitors I interpret as follows:
      C271P = Ceramic Disk 270 pf  (27 x 10^1)
      M104 =  Monolithic 100000 pf [0r 0.1uF]  (10 x 10^4)
      4.7/50 = 4.7 uF at 50 volts DC maximum
    These fall in with "standard value" components.

D)  The electrolytics in the signal path are large values because of the low values of impedance to ground following the capacitor (drop less intended signal across the cap, and more across the load) -- remember capacitive reactance = 1/(2*{pi}*F*C) - bigger capacitance, lower effective resistance.
Electrolytics are the most cost effective means of achieving these aforementioned larger values of capacitance.

E) The third stage is a recovery stage for the tone shaping network.  There is always a fairly large signal drop in a passive tone stack - anywhere from 6 dB to 30 dB of loss occurs.  Remember a passive tone stack CANNOT add to the signal, can only "take away" parts of the signal (when bass is at max, treble is being cut - when treble at max, bass is being cut).  This stage also provides a stable impedance for the tone shaping network to 'work against" - crucial to proper tone network operations.  The stage cannot be taken out without drastically affecting the tonality of the output.

Finally, if you are keeping the LEDs for the "overdrive" - you cannot eliminate the second stage either.  This stage is the buffer for the overdrive, which in the original design is used as a boost by increasing the gain (as mentioned in a prior post).  Plus, C7 in this stage does just a bit of tone shaping (a treble cut) in preparation for the overdrive and the tone stack.

I hope this helps your understanding  :dbtu:

Schematics and Layouts / Re: Vox pathfinder stripped
April 11, 2012, 08:28:14 PM
You are correct in that the "second stage" is always in the circuit.  However, when the boost switch is NOT engaged, Q1 (the FET in the feedback path to the second amplifier stage) is turned "on" making the feedback resistance equal to approximately 22 kohms (value of R8) - effectively superceding the value of R6+R7.  The gain of this stage (as all op amps) is determined by R(feedback)/R(input)  - the input value in this case is approximately the value of the 22k resistor right before the capacitor attached to pin 6 of this IC.
All that being said, with the boost switch "off" the gain of this amplifier stage  is approximately unity or "1".  Turning the boost switch "on" shuts OFF Q1, and the gain then becomes  (150k [R6] + 470k [R7])/22k [R5] or (to work the math) approximately a gain factor of 28.  (HUGE difference!)

Indeed the LEDs work in the Overdrive drive function.

The Tremelo is the center portion of the circuit, which is coupled to the preamp by the optomodule (note between R19 and C18).

So, all that being said -  and to keep all you like, losing ONLY what you don't the modifications become easier:
Remove Q1.  Place a wire in the spots formally occupied by Q1 drain and source.  Remove the optocoupler,  run a connection between R19 and C18 (where two pins from the optocoupler used to be.

Now, you have the preamp without the boost, and without the tremelo.  In circuit form, you have set the Trem setting to ZERO, and the boost switch to "OFF" permanently.

Good luck and happy building!
Schematics and Layouts / Re: Vox pathfinder stripped
April 11, 2012, 06:41:13 AM
Just a quick observation:  The Schematic (attached) shows only one input CHANNEL, it only goes into distortion when the "gain" control is "cranked up".

That being said, I suggest the following changes on the "top row": from left to right - cut right after R5 (22k) and right before C11 (4.7/50).  Drop the entire section cut off your new schematic.  Join the two sections of the "top row". Eliminate LED1 and LED 3.  Combine the values of R15 and R16 into one resistor of 1.5 kohm. Cut right before R19 and after C18. Drop the "cut section" out. Join the two sections together.

These mods lower the overall gain, eliminate the crunch diodes and remove the tremelo from the preamp.  It may need further "tweaking" - but you at least have one opinion on what to do now!

Good luck!
Good day!

I have attached another version of my "Buddybox".  It utilizes a single ended power supply with a discrete rail splitter circuit (not my idea, but found elsewhere).  If R21 is carefully matched in value with R3, and likewise R22 with R21 in the splitter circuit, the voltage split will be maximum closer to even.

The overall gain of the circuit is about 2.

This mixer was built to accomodate piezo pickups, passive coil pickups and also line level signals such as put out by a keyboard.

If only using one channel, such as to interface a piezo pickup to another system, be certain and turn the volume down to zero on the other channel - to eliminate noise issues.

One can utilize a small line operated transformer for the power source, or one can hook the unit up to a "wall wart" supply as shown.  The rail splitter can supply as much as 100 milliamps of current, thus, the power supply transformer or wall wart need only provide 200 milliamps or more of current capability.

Thanks to Sijosae for the rail splitter circuit!

I found that the OPA2134 gave the most "headroom" for the signals,  the circuit will work with a TLO-72 for U1 and a NE-5532 for U2.

Hope ya'all can use it!
The transmogrifier (passive mixer cable) attached
And now, the Mic/Guitar mixer circuit:  Independent control of either: two guitars, two microphones with 1/4" outputs or one guitar and one microphone into a single amplifier.

The TL072 allows either a high impedance (1.35 Megohm for guitar) or a lower impedance (10kohm) for microphones. When in "Guitar" mode, one of the gain resistors in the feedback loop of the amplifier stage is shorted,  resulting in a lower gain -- suitable for the electric guitar output levels.  Flip the switch, and this short is removed, the lower input impedance is engaged and now it is suitable for a microphone level input.

The overall gain of the circuit in "guitar" mode is a maximum of 2.5 and a minimum of zero (this has been tested, and seen on the oscilloscope).

Output impedance is only 5000 ohms, low enough to drive virtually any amplifier quite well.

I've been using the unit since March, primarily in "guitar and mic" mode - working great!


Power supply notes:

In this case,  the power supply regulators are 7815 and 7915 in the standard usage thereof (as opposed to 7805/7905 used in an "adjustable regulator" configuration.

The 1000 uF capacitors following the regulators serve as "stiffeners" to the regulated DC output.  These provide a current "sink" to buffer the regulators from rapidly changing load currents - the capacitors can dump current faster than the regulators, thus improving the overall load regulation of the supply.


The schematic of the Guitar Splitter attached
To Phatt -

I've tried twice to post with an copy of the schematic attached, and get server errors --
If my other two posts make it, please forgive this third post with the same information.

The regulators are NOT LM7815 and LM7915,  they are LM7805 and LM7905 in the power supply -- not an error or a typo.  The tabs of the regulators are ground isolated, and the R8/R7 and R9/R10 ratio brings the output voltage up to +/- 15 volts as shown.  Put a LM7815 and LM7915 in the circuit, it will NOT work.

The diodes D5 and D6 prevent the possibility of there being a greater voltage at the output of the regulator than at the input of the regulator, this is the reason for the current practice specificaiton of 10 uF on the output - to prevent that possibility.  IF these "bypass diodes" are in place, capacitance at output of regulation only reduces any ripple not rejected by the regulator, and the capacitance value has NO maximum. IMHO - the less ripple the audio circuits have to reject, the better! 

From a practical standpoint, I usually build my supplies with 4700 uF on the input side of the regulator and between 330 and 470 uF at the output side of the regulator, and usually do not use the bypass diodes with these values of input and output capacitance. The ratio of 10x the capacitance on the input side vs the output side, with the "bleed current" seen through the voltage setting resistor network, is usually quite sufficient to prevent the circumstance of the voltage at the output of the regulator being higher than the input voltage to the regulator from EVER happening.    It is far more important to make certain that you have RF bypass caps (0.1uF) on the input to ground and output to ground side of the regulator if you have the regulator located ANY distance away from the input and output main filter caps, than to have a "maximum" output filter cap value of 10uF.

You need to re-check the specifications and application notes on these, and you will find both sets of information I give above therein.  I have been building regulated power supplies in this fashion since 1979, and haven't had problem one with this design yet.

The DC input resistance of the circuit is 2.7 megohm,  however the audio IMPEDANCE of the circuit is 1.35 Megohm.   I have been using this unit continously since January, and have had zero noise problems, and have used this combination of twin 2.7 megohm resistors with a DC blocking cap on the input of several devices for almost 3 years without problems of noise.

I'm sorry I didn't respond earlier, I had changed E-mail addresses and did not update the change on this forum,  I have rectified this oversight.

I like to use my main Solid State amplifier (posted under "outboard amp") and a tube type Fender "Champ" clone I built at the same time.  To facilitate this, I built a splitter (schematic attached).

The power transformer came out of a junked HP 310.  It was mostly from my junk box, and is indeed a bit over designed.  My idea was to have the guitar signal split without any loss o f signal to either amplifier, or loading down the guitar.  Thus the input impedance of 1.35 Megohm, and the use of the opa2134 FET input opamp.  I had some 2134's, but I am certain it would work as well with a TLO72. :tu:

The power supply is overbuilt, but again, I used parts I had on hand.  The use of the split power supply allowed the use of noninverting input, without concerns of trace impedance variations used by an "artificial ground".  The positive and negative regulated power supply is rock solid, and dead quiet.  In fact, the entire mixer is "overbuilt". ;D (But then again, I started by opening my junkbox, and seeing what there was available for use in making my mixer.) 8)

The upshot is that the combined sound coming out of both my amps at the same time is just simply AWSOME!   :tu:

The splitter circuit could be used for virtually any audio circuit with a maximum signal level of about 2 volts p/p.  The power supply voltages are the limitation on the maximum signal used.  Respectfully submitted for your edification!

The second item is the "Transmogrifier".  It is a passive mixer cable assembly that I and many others locally use to play our MP3 players through our amplifiers.  The resistors provide a channel isolation, preventing possible shorts from one channel to the other.  Just a combiner cable without the resistors may work fine, but with the "transmogrifier" the system is virtually "bulletproof" to enjoy my music through my existing mono amplifers.

The input impedance of most of my amplifiers ranges from 50,000ohms to over 1 million ohms.  The signal lost across 91 ohms is insignificant compared to the voltage present at the amplifier input.

Just thought I'd share and start this "gadget thread".

I'm working on a guitar mixer, allowing two guitars to be played through one amplifer -- without loading either guitar down -- more on that later!

Anybody out there got some gadgets they use that answer their little needs like these do for mine? :)

Hello Again!

Well, after much work and testing -  the amplifier is completed and functioning to my satisfaction!  Originally planned as an "outboard" amplifier, its sensitivity is such that I can plug the output of my chorus pedal directly into the front end, and have very satisfactory results.  To say I am well pleased is an understatement.  This amplifier has become my new favorite for "living room" and small gig work.  It has a high end of about 45 watts, and is "transparent" in that it does not color the sound - suitable for my guitar work, or, with a mic preamp, vocals work.

Here was the integration of a handful of parts and modules into the case and then a complete combo amplifier is born.

First, I began with the power supply.  In the first image (pwrsuplie0.jpg), you can see the basic configuration of the transformer bolted to the bottom of the amplifier.  The +15V filter diode and filter caps on the right, and the +/- 12 V supply on the left.  I used a silicone based caulking compound to mount the capacitors - eventually dries clear.  I added a dual duplex outlet to the back panel, this can bee seen in the second image (inner4lj7.jpg).  You will note that I used #14 solid wire to connect the duplex to the "IEC" input power plug.  I did not fuse the "built in extension cord", but will run the power for the amplifier proper through the fuse. The image (inner3ix8.jpg) following this shows the power supply assembly in the final build configuration.

Next, I integrated in the preamp and control assembly.  This is seen in the next image (inner1es3.jpg).  You will observe that I routed the +/- 12V through a shielded cable, that the control panel (aluminum) is tied to ground and the frame of the power transformer is tied to the ground plug of the IEC connector.  I found out through trial and error the ground from the +15 Volts is tied to the ground from the +/-12V only at the board.  A second connection between these two grounds produced a ground loop buzz.  This goes along with the single point "star grounding" philosophy that is often mentioned on other DIY audio sites - ground loops can give you fits!

I ended up not installing a power LED indicator, the position of the switch tells me whether the unit is on - and that eliminated some additional wiring "hanging out" inside the amplifier to pick up hum.  (Might or might not have occurred.  In my case, the "power on" indicator was not an essential item - so I left it out).

Next, I installed the power amplifier modules (seen in inner2jm0.jpg).  You will note a couple of things: 1 - the outside modules are the FK-607 modules from my kit and the center module is the twin TDA2003 amplifier previously build. 2 - the input line for the power modules comes from the preamplifier board through a shielded pair cable.  The positive and return lines are connected as normal, but the outside shield of the wire is connected only at the preamp ground end - to minimize stray signals, and prevent another ground loop.  Finally, note the heatsink orientation and the fact that the center module is mounted by the heatsink - to reduce strain on the TDA-2003 chips - that much heat sink is HEAVY!
You will note that each speaker has its own power amplifier module, per the original design.  It is critical to have the speakers all in phase (the high side of the amplifier output to the "+" terminal of the speaker, for example).  If one of the speakers is wired backwards to the others, there will be some sound cancellation.  By wiring all three effectively  in parallel, they reinforce one another, for an excellent sounding output.

Ok, I've got all the innards put together and ran final tests to make certain all is working as planned - now as to the final look at the outside.

The front panel is shown in (dout2pr8.jpg).  Very basic, classic and clean design.  The Pekingese on the front is the "Astro-Peke", my dog and mascot to a small repair business I started and closed this past year.

The rear panel is shown in (dout1uy2.jpg).  The on switch is on the far right, the input between the switch and the volume control and the buffered output port on the far left of the control panel.  A fuse holder, IEC power in and duplex AC outlet assembly complete the rear panel.

The wood is sanded and covered in 3 coats of clear polyurethane - the sound is awesome. The design is for crystal clear sound, no distortion at all and it DOES shake the windows when turned up!

A final word:

As you can see, there is much more to building an amplifier than just getting a schematic.  Case, assembly, lead layout and control layout as well as speaker placement and case materials and construction all affect the final sound.  I could alter the sound slightly by porting the cabinet, but I like it with the "closed back" sound, and have no intention of changing it.

I hope reading this process was informative, and gives you an idea as to what goes into designing and building a combo amplifier - solid state style.

As with any of my designs, this unit is not totally optimized out as far as performance,  However, it is designed to run trouble free for a VERY long time, and it fits my needs for a combo amplifier perfectly.

Have a great day, and keep on making music!
The Newcomer's Forum / Re: Pre-amp power question
January 26, 2009, 10:23:55 AM
Another option for a regulator is to use an LM317 style "adjustable" regulator. Using it, the tab and "ground" termination are "lifted" from ground and thus not an issue for higher than 35Volt input.

Check out the data sheet from national semi for ideas

Hello Again!

I've been putting in some hours on this project in this holiday season, which is why the posts came closer together!  This post, I'll show the front and back panel work.

First, I cut out the speaker holes and drilled the mounting screws for the speakers in the front panel board.  (see barfrnthw8.jpg)  below.  Next, I installed mounting bolts - 8x32 x 1.5 inch - with the screws bolted to the board, then slid the speakers over the mounting bolts for a test fit (see skrbrfntmy2.jpg).

I then pulled the speakers off, and sprayed the front down in black so that it would not show up as bare wood under the speaker grill cloth. (blkfrntcl8.jpg).

For the grill cloth, I had a plastic mesh I had picked up at some surplus place for a song.  I have used colored burlap and other "open mesh" fabrics for grill cloth before.  I wrapped my mesh around the speaker panel, installed speakers and slipped the assembled panel into the box for a test fit. (See  skrinstbktn6.jpg)  The grill cloth was stapled to the back side of the panel after it was pulled around the edges.  The next picture (spkinstfntwg9.jpg) shows the front view of the box with panel in a "test fit".

Now, I move on to the rear panel.  (See backplatenz8.jpg) I carefully measured and cut holes for the control panel assembly (top middle), the power input jack and fuse holder (left bottom) and for the two duplex 115 receptacles with outlet cover (bottom right).  Finally, I drilled three mounting holes on each side to screw this backplate onto the main box for assembly.  I sanded the unit, put a polyurethane clearcoat on it, sanded lightly, then I put a coat of black over the clear, sanded again - removing the black from the high spots -  and then a final clearcoat.  The effects on the backplate appearance are seen in hndlviewyg6.jpg.  I have installed the Duplex assembly onto the backplate to assist me in placement of large components within the box (specifically the power transformer).  You also see the handle on the top to the amplifier box.

In feetfitsb0.jpg, you see the "feet" I attached to the box, as well as the four mounting holes for the power transformer.  The power transformer, being both heavy and the largest component to go into the box, needed careful placement to keep it physically away from the mains lines as well as not interfering with any controls or operation of the amplifier.  From the holes, you can see I elected to put the transformer in the middle of the bottom panel of the amplifier - keeps center of gravity low as well.

Finally, for this post, in spekerfitcg8.jpg, you see the method for insuring the speaker panel is affixed within the main box.

From here, it is a matter of taking our handfuls of parts and modules and integrating them into an operational amplifier assembly.

That comes NEXT post!
Schematics and Layouts / Re: "Outboard" Amplifier Project
December 30, 2008, 07:46:30 AM
Sorry folks - I just noticed the date is over year off on my digital camera!, I never reset the date after last battery change -- oh well!  :loco