Welcome, Guest. Please login or register.

April 01, 2020, 02:56:50 AM

Login with username, password and session length

Recent Posts

Author Topic: the K15  (Read 11670 times)

benzer

  • Master
  • ***
  • Posts: 63
  • Chip Points: 0
    • View Profile
the K15
« on: September 21, 2007, 07:03:20 PM »
howdy folks, i decided to build this simple amp as a first step into building amps,,its a dean markley chip amp but the preamp is pretty cool,its capable of driving 10 watts into a 4ohms speaker.
i included the circuit here for anyone who's interested
anyways i have some points wich are not so clear to me in this circuit,
1/wuts the R1 for ?
2/about the caps its written for example c1= 473M,,, wut the hell is M?? microfarads?and some caps like c13 is 22/16 ,,whats that stands for? 22microF/16 volts?????
3/i didnt get what the rocker switch connet n disconnect,,can anyone discribe the way it works(what terminals it connects n dont)?
4/about the pots ,, wuts A250K B250K ?? wut ranges should they take?
5/can i get rid of c19? or its better to have it?
6/wuts the point of having R10 and C11 ???
7/any comments about what kind of speaker and op-amps are suitable for this circuit is pretty welcome
.....thanks already for any answers,,hope this amp sounds good
Good OL' BeN

benzer

  • Master
  • ***
  • Posts: 63
  • Chip Points: 0
    • View Profile
Re: the K15
« Reply #1 on: September 21, 2007, 07:06:20 PM »
here is the amp
Good OL' BeN

teemuk

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 1074
  • Chip Points: 282
    • View Profile
Re: the K15
« Reply #2 on: September 21, 2007, 08:23:36 PM »
1. Since I see no shunting capacitor I believe its sole purpose is to limit current and thus protect the input stage. Anyone?

2. IMO, this is again FAQ material: First two digits=value (in picofarads), third digit=multiplier (or number of zeros i.e. 1=10, 2=100 3=1000 etc.)
Letter code marks tolerance:
C: +/- 0.25pF
D: +/- 0.5pF
F: +/- 1%
G: +/- 2%
J: +/- 5%
K: +/- 10%
M: +/- 20%
P: -0% / +100%
Z: - 20% / +80%

So, for example, 330P stands for 33 picofarads -0% / +100%. 473J stands for 47 000 pF (0.047uF or 47 nF) +/-5%. Simple?

3. They obviously tried to imply that the switch is integrated to footswitch jack. Anyway, you can picture the “block” moving left to right (and vice versa) thus connecting the center terminal to either the left- or rightmost terminal. When drive is on, “input” comes trough R6 and C5 and the diodes are connected to feedback loop to clip the signal. When drive is off the diode clipper is removed from the loop and input comes through R5 & C4, which obviously reduces gain.

4. The letter marks the taper.
Old code: A=linear, C=Logarithmic (audio) and F=Antilog.
New code: A=Logarithmic (audio), B=Linear, antilog pots are no longer in production

5. Theoretically you could get rid of it. However, I think they have a good reason for putting it there. Anyone?

6. Same as having R9 and C10: It’s an RC supply filter. Essentially it lowers the supply rail a bit, filters it and decouples the opamp supply from the supply of the output stage’s power opamp. This reduces interference between different stages of the circuit.

7. Opamps can be any generic model (i.e. 4558, as implied by the schematic). Use dedicated instrument or pro audio (PA) speakers. Do not use consumer grade HiFi speakers.
« Last Edit: September 21, 2007, 08:49:34 PM by teemuk »

benzer

  • Master
  • ***
  • Posts: 63
  • Chip Points: 0
    • View Profile
Re: the K15
« Reply #3 on: September 22, 2007, 06:26:40 AM »
thnx a lot man, u answered all my questions..
i was thinking to do any modification to the circuit but it looks pretty cool i dont wanna change anything
ill try to omit the c19 and see how does it affect the sound,, i dont think its more than a protection against dc .
since there are q's can be included in a FAQ ,why dont u guys make one? who is in charge here?since it saves a lot of questions its a good idea to go with.
again there are some caps in the circuit thats drawn different like c15
whats that shape means? and are they ufarad/volts ?

 
Good OL' BeN

teemuk

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 1074
  • Chip Points: 282
    • View Profile
Re: the K15
« Reply #4 on: September 22, 2007, 07:24:50 AM »
Yep, in that case those are uF / volts. If you look closer you will notice that they have a marking, which indicates the correct polarity. The alternative shape simply distincts them from non-polar capacitors. I guess both IEC code and a normal format of uF were used simultaneously because polarized caps are often electrolytics, which have the capacitance printed to side of them in the latter format. Non-polar ones tend to indicate the capacitance using IEC code.

If you omit C19 be sure to measure DC offset in the output before attaching a speaker. I have feeling it may vary when treble or bass control is adjusted.

By the way, if you make R8 (5.1K) in the tone control variable you got yourself a "mid-range" control. I think 10K linear potentiometer wired as a variable resistor should do the job.

Other mods: I would hookup two diodes from IC2 output (pin 4) to rails like this: Diode #1 cathode connects the positive rail, anode connects the output. Diode #2 cathode connects the output, anode connects the negative rail. This will protect the output from inductive peaking. In normal conditions the diodes do nothing but when the output signal exceeds that of the rail voltage (plus voltage drop over diode) the signal is shunted to supply and clipped. These diodes must be able to take some current: For example, 1N4002 is a proper device choice for the purpose.

Hook a similar diode arrangement to the pin 5 of the input opamp and you have yourself an overvoltage protection, which will protect the input stage. Cheap amps often lack these simple protections. 1N914 does fine here since you need less current handling capability than in the output.

Bypasss supply pins of the opamp (pins 8 and 4) to ground via 100 nF capacitor as close to the pins as possible. These caps will compensate the inductance of wiring and act faster to transient current draw than the main 220 uF caps. This will increase stability and decrease the possibility of the opamp oscillating by default. You can also hookup pins 8 and 4 together with a third 100 nF capacitor.

As shown, the headphone out feature is rather useless. You need to wire it in such a way that plugging in the phones will disconnect the speaker. There are phonejacks that come with a suitable integrated switch for the purpose. It is strange that they chose to take feed for "line out" before the capacitor.

Yep, it looks like a very basic, simple and cheap amp. Expect it to sound like one as well.
« Last Edit: September 22, 2007, 07:29:46 AM by teemuk »

benzer

  • Master
  • ***
  • Posts: 63
  • Chip Points: 0
    • View Profile
Re: the K15
« Reply #5 on: September 22, 2007, 08:10:59 AM »
Quote
if you make R8 (5.1K) in the tone control variable you got yourself a "mid-range" control
thats a nice shot,,but im not pretty sure of the base range and the midrange in this circuit as long as the -3db of the highpass of r4/c3 is about 372 hz ,,,why on earth they cut the frequencies below? wouldnt that affect the bass control,,lookslike it needs too much boosting to have base ,,, :o
same thing for other c2/r4 a lowpass with about 300,000.0 hz as a cutoff ,,why that much? isnt better to make it somthin about 30,000.0 hz ??????????????
you got also the lowpass of the volume control pot with the c6,,the band pass would get smaller or bigger as we vary the volume. wuts the point??protection against high amplitudes at high frequencies?could be?

in general the whole circuit looks fine to me but the cutoff frequencies of the filters look a lil strange as i indicated above.

Quote
Hook a similar diode arrangement to the pin 5 of the input opamp and you have yourself an overvoltage protection, which will protect the input stage.
what should cause an overvoltage at the inputs? too much gain?


Quote
As shown, the headphone out feature is rather useless. You need to wire it in such a way that plugging in the phones will disconnect the speaker. There are phonejacks that come with a suitable integrated switch for the purpose. It is strange that they chose to take feed for "line out" before the capacitor.
yea you are right and i decided to not include this headphone plug at all,about having the feed before the cap ,,mmaybe headphones'speakers have their own protection against dc,,or low dc voltage would do no harm.

Quote
Yep, it looks like a very basic, simple and cheap amp. Expect it to sound like one as well.
yea it is but isnt chip amps supposed to sound better than discrete ones?
why would it sound cheap? a chip power amp with an op-amp preamp
,,i guess that should sound fine...maybe using the TL072 would make it better
Good OL' BeN

joecool85

  • SSGuitar Admin
  • Legendary
  • ******
  • Posts: 3230
  • Chip Points: 988
  • SSG Creator
    • View Profile
    • thatraymond.com
Re: the K15
« Reply #6 on: September 22, 2007, 08:18:22 AM »
If you want a really killer small amp, build a Dean Markley K-20X.  The schematic is still on dean markley's website.  I bought one as my first amp 7 years ago and it is still my favorite practice amp.  I hooked it up to a 10" dean markley cab (it comes with a built in 8" speaker) and it is loud enough with only 14w RMS to keep up with bass and drums.  Plus it has excellent tone.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

teemuk

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 1074
  • Chip Points: 282
    • View Profile
Re: the K15
« Reply #7 on: September 22, 2007, 10:15:52 AM »
They may have introduced so much low frequency cut to make the overdrive sound less muddy, who knows... It can be a mistake as well. (Those Dean Markley schematics seem to be full of them). The low-pass filter becoming more prominent at higher volume settings may be beneficial in removing "harsh" high frequency content from the signal at higher volumes. Anyway, I don't like the arrangement that drives an opamp stage with negative gain (attenuator).

The "overvoltage" protection protects (or at least tries to protect) the input from sources that introduce a too high input signal amplitude. For example, if someone decides to "experiment" by hooking up a power amp out to the input of this amp. ...Or something. Even static electricity can cause high voltage spikes. These are poison to the input stage of the opamp.

About that output cap: Headphones do not have a DC protection (they're just speakers) - and yes, low DC voltage, i.e. few millivolts, does no harm but it does no good either: It can't move the speaker cone so it only increases the heat dissipation of the voice coil.

On the other hand, many "line in" inputs are AC coupled so they can be fed from a point before the coupling cap.

Still, I don't know why the included the output capacitor (if it's a hi-pass it would have been cheaper and easier to implement it at the preamp side). Like I stated, I assume it is a DC protection since messing up with the tone control values may upset the DC offset of the output. They still could have compensated that by other means, i.e. by AC coupling the tone control from the power amp.

Anyway, since the output has nearly zero DC volt potential that capacitor needs to be non-polar. Those are expensive and rather unreliable components. They are also lot less linear than ordinary polarized electrolytics but using such a low value capacitor will cause distortion already. In HiFi amplifiers this arrangement would not be acceptable. In guitar amps where linear reproduction is not a demand it's, well... It doesn't look like good design. However, neither does the whole amp so why bother with guessing the designer's motives.

The tone issue: The chip amp will pretty linearily reproduce the signal fed from the preamp (but with higher power). That is it's function, which it does a lot better than average discrete amps. However, the preamp is the problem: Its basically just two gain stages, one with switchable gain and diode clipper. These drive a fender-type tone control circuit. Don't expect any miracles from such a simple arrangement: Likely the clean sounds are good (they can be even excellent) but I think the O/D channel may have characteristics of a crappy fuzzbox. That's because the circuit practically just puts a couple of clipping diodes to the signal path. Such simple arrangement nearly ever sounds any good.

Alternative opamps can't make circuits to sound any better if the voicing is done badly in the first place - at best they just decrease noise or something.

Tone is highly subjective issue so it's pretty vain to comment or speculate about it. Let's just say that I'm anything but impressed by this circuit. By default, these low power practice amps are results of little money being spent on their design. They are not especially "high-quality". Complement them with a cheap speaker stuffed in as virtually small cabinet as possible and you got yourself an amp you will begin to hate after few years. Simplicity is a virtue all right - but not when it has an effect on tone, performance and features. IMO, the amp circuit suggested by Joecool85 looks way better than this one, allthough it still contains many of the flaws seen already in this circuit (no supply bypass for opamps or the power amp chip, opamp stages running at negative gain, that strange output cap).

Since you are obviously after a DIY project that will provide a satisfactory outcome why not do it right from the beginning? Throw away the idea that the amp schematic should look simple. Learn a new way of thinking. With little and simple mods those amps can be improved dramatically from the original. You can put in features, like footswitches, channel indicators, FX loops etc, that they never even decided to implement (to such a cheap amp) in the first place. Those still can't change the fact that these amps are far too loud for bedroom use and far too silent for serious practice or gigging with a band. Anyway, it's a start. If you make a good preamp now you can later sub the power amp section with something that has higher output power. If you build something cheap you will eventually find yourself hating and never using it. - And likely you just throw the thing away in a year or two. Trust me, that has happened to me many times.
« Last Edit: September 22, 2007, 10:21:14 AM by teemuk »

benzer

  • Master
  • ***
  • Posts: 63
  • Chip Points: 0
    • View Profile
Re: the K15
« Reply #8 on: September 22, 2007, 07:41:01 PM »
Quote
the preamp is the problem: Its basically just two gain stages, one with switchable gain and diode clipper. These drive a fender-type tone control circuit. Don't expect any miracles from such a simple arrangement
whats the bad stuff? the negative gain? how about modify that for a positive gain, and modify the resistors values,,would it help?
or the bad thing is the fendertype tone control ?? how about making the tone control an active circuit ,,any better?
i dont wana talk now about protections as much as i wanna talk clean sound,,
about the negative gain again all i know is that it shifts the phase 180 degrees,,so wut distorion would it make if i put two negative gain stages like its included in the design so i have the exact copy of the input signal with positive gain(180+180 = 360 = 0) any wrong with that?
 
good idea about ac coupling the power amp from the tone control ,lookslike we got rid of the cap this way.nice shot :tu: .

Quote
Since you are obviously after a DIY project that will provide a satisfactory outcome why not do it right from the beginning? Throw away the idea that the amp schematic should look simple.
well i was goin for simplicity cuz its gonna be my first built amp as i told you before ,,and yes nothin wrong to apply mods to the design and i will,like changin some resistors values(filters) and have the midrange pot u suggested and bypassing the op-amps with diodes also ill think of better supply,use the TL074 ,i love this IC's s;ew rate,, <3)lol,thats what i think of now.
,if you suggest me another circuit that sounds better than this and is fine for a first build Please let me know.
maybe i should go for joecool's suggestion,.thanks joe for the notice n thanks teemuk for your mods.
Good OL' BeN

joecool85

  • SSGuitar Admin
  • Legendary
  • ******
  • Posts: 3230
  • Chip Points: 988
  • SSG Creator
    • View Profile
    • thatraymond.com
Re: the K15
« Reply #9 on: September 24, 2007, 07:38:41 AM »
I find my K-20X perfect for bedroom use but only because it stills sounds good turned down.  It is quite loud with a good speaker.  And the built in overdrive is one of the best overdrives I've heard in a while, best for old school rock tones and not much else however.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

teemuk

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 1074
  • Chip Points: 282
    • View Profile
Re: the K15
« Reply #10 on: September 24, 2007, 09:30:34 AM »
whats the bad stuff? the negative gain?

Oops. My bad: I should have used a term "less than unity gain", which is more correct. That is prone to instability and opamp manufacturers generally suggest that such configuration should not be used. Why build an active attenuator anyway...

benzer

  • Master
  • ***
  • Posts: 63
  • Chip Points: 0
    • View Profile
Re: the K15
« Reply #11 on: September 24, 2007, 11:20:19 AM »
Quote
Oops. My bad: I should have used a term "less than unity gain", which is more correct. That is prone to instability and opamp manufacturers generally suggest that such configuration should not be used. Why build an active attenuator anyway...

sorry but i dont see any less than unity gain configuration.
do u mean that the vol pot of the second stage can cause a unity gain?
even though ,,at this level the sound would be very very low so it wouldnt affect much.

well anyways i think ill go for the K-20X as long as joecool thinks that its a GREAT AMP...thanks joe and any of you guys wanna talk its circuit? cuz it looks impressing to me.
Good OL' BeN

teemuk

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 1074
  • Chip Points: 282
    • View Profile
Re: the K15
« Reply #12 on: September 24, 2007, 03:19:03 PM »
Well, isn't it obvious? Roughly the gain of the second stage is determined by VR1 (potentiometer) divided by R5 or R6 (depending on the "drive" setting). This is the basic gain math for inverting opamp configuration. Now, when VR1 is set to lower value than R5 or R6 (again depending on "drive" setting the gain becomes less than unity. For example,  25K/33K = 0.75. Since VR1 is logarithmic this would be the gain with volume at five on "clean" channel. Any lower volume setting would also be attenuating. So, when it comes to clean channel it is pretty realistic to expect that you run this opamp stage mainly as an attenuator, Drive channel is a bit better since R6 is considerably lower than R5.

I don't see what a possible risk of oscillation has to do with volume levels.

Anyway, it is obvious that the designer(s) got away with this. However, if you end up with a sloppier layout or an opamp that's not particularly happy about running it with less than unity gain setting you might end up building a medium power oscillator. I'd rather try to avoid such scenario in the first place.

The other amp...

K-20X looks pretty allright since it at least tries to "voice" the clean and "O/D" channels a bit differently. It still has the same less than unity gain "flaw" for clean channel but I like the way how the gains for different channels are arranged. Since the circuit is not too simple it will also lend itself to more extensive modding if you don't like its tone.

Some other comments...

- The lack of feature to switch channels with a footswitch sucks. If I were you I would spent some time in engineering this feature because it's extremely useful
- The input impedance is, IMO, too low (only 220K)
- It has those protection diodes in input and output I was talking about earlier (and in the particular schematic they at least managed to draw them correctly). Good.
- In case you didn't know, that dual LED can be replaced with discrete red and green LEDs. Using different colors causes asymmetric clipping, which is pretty much the intention. I would try this also with ordinary diodes and see which ones I prefer. The latter might create a bit more aggressive distortion, although you loose some volume in comparison to LEDs. If you like both setups make them switchable.
- Still has that output capacitor. For some reason its larger now...
- The amp uses a current feedback scheme that increases output impedance "to mimic tube amps". It also allows creating a "damping" control if you put some variable series resistance to R14. Very popular setup nowadays.
- Opamps and the chip amp still lack the close vicinity rail decoupling that decreases risk of oscillation
- The schematic has incorrect labeling for bass and mid-range controls
- They at least have tried to correct DC offset versus tone control setting issue by selecting R13 = R12 and using R11 to separate the chip amp input from the tone controls (besides its function as LP-filter). I would still AC couple these two circuits.
- The headphone jack feature of this amp is finally usable since it disconnects the speaker. Value of R18 is not shown
- Make that mains switch to switch off both "Line" and "Neutral". It's safer that way.
- The Zobel: R17: Use non-flammable, medium power (i.e. 1 - 5 W) resistor. C13: Use polypropylene self-healing cap. If the amp oscillates these parts can save the chipamp but will take a lot of punishment.
- The volume control for drive channel is linear so it might be a bit inaccurate in controlling the volume level (i.e. Volume rapidly changes to very loud even at low settings but after that the control seems to have less effect). It's a cheap and annoying trick that is used to fool people to believe that the amp is louder than it actually is. Does your amp suffer from this issue Joecool85?

I guess that was pretty much all I have to say about the K-20X circuit.

benzer

  • Master
  • ***
  • Posts: 63
  • Chip Points: 0
    • View Profile
Re: the K15
« Reply #13 on: September 24, 2007, 06:23:50 PM »
some points about the k-20X :
1/ that input configuration R1 R2 C1 C19 , thats a second order bandpass filter ,,well it doesnt make any phase distortion as long as the frequencies it passes are in a very big bandwidth ,,
but still dont know why use this kind of configuration.

2/the cutoff freq of the lowpass C2 R4 is about 225 khz ,,why that much???? couldnt it be just about 40khz???

3/ nice configuration for the second op-amp,a third order filter for the overdrive,but, are the switches SW1A SW1B both switch in the same time?

4/ why use a lowpass to couple the tone control with the chipamp??

nice idea about using two different color leds teemuk ,ill try to include it and make a switch for this,,also the footswitch.

Quote
It still has the same less than unity gain
cant we modify the values of C3 R5 to solve this? and let the pot take lower values than before
Good OL' BeN

joecool85

  • SSGuitar Admin
  • Legendary
  • ******
  • Posts: 3230
  • Chip Points: 988
  • SSG Creator
    • View Profile
    • thatraymond.com
Re: the K15
« Reply #14 on: September 24, 2007, 06:51:44 PM »
- The volume control for drive channel is linear so it might be a bit inaccurate in controlling the volume level (i.e. Volume rapidly changes to very loud even at low settings but after that the control seems to have less effect). It's a cheap and annoying trick that is used to fool people to believe that the amp is louder than it actually is. Does your amp suffer from this issue Joecool85?

I guess that was pretty much all I have to say about the K-20X circuit.

Actually yes it does.  It doesn't really get any louder past 1/2 volume or so, maybe 2/3.  It also picks up volume very quicky on the low end of the knob.  For instance, 1/4 volume is relatively loud.  I've gotten use to it, but if the pots wear out I will probably replace them with the log pots it should have had to begin with.

I'd also leave the OD circuit as is, at least at first.  I love the tone it puts out, it sounds sweet.  In fact, it's nice enough that a couple Peavey fanatics I know are jealous of my practice amp.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com