| Input impedance / load? K200 [message #21077] |
Thu, 07 August 2014 21:47  |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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I've searched the forum here, but haven't found much that deals with the input impedance of the Kustom amps...which in turn is the load placed on our instruments.
One of the things I do on the side is building very nice sounding DI units (direct boxes) for recording guitar and bass. Over the years, one thing I've learned is that passive guitar and bass pickups don't like being loaded very heavily. I try to stay above 500k ohms for the load if the player likes using passive pickup instruments. With an active DI 1meg ohm isn't a problem.
As I've been looking at the preamp sections of the K200 and K400 amplifiers, it looks like most of them would load the instrument very heavily...like around 50k ohms. The PC102 being a possible exception at closer to 1meg.
Does anyone have any thoughts about this? Passive guitars can sound pretty muddy when loaded down to 50k. I'm wondering if a change of the first input resistor to a higher value might be a good idea?...or perhaps a bad one? Has anyone gone here before?
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: Input impedance / load? K200 [message #21080 is a reply to message #21077] |
Fri, 08 August 2014 06:48  |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Yes, I have noticed that also, and like you say putting in 500k pots into a axe that has the norm 250k will increase the volume some and the highs which I have found with some single coil pickups can be a issue!
My main guess as to why they went that route was due to the fact of noise being made thru higher load resistance (resistor noise) in conjunction with transistors that where already on the noise side even when being state of the art types for back then!
Also note that on some amps there is a built in brite boost circuit that is always on.
[Updated on: Fri, 08 August 2014 07:10] Report message to a moderator |
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| Re: Input impedance / load? K200 [message #21081 is a reply to message #21077] |
Fri, 08 August 2014 10:41 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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Glad you've noticed the same thing Steve!
Last night, when testing my revitalized K200, I tested a couple of my (passive) Fender P-Bass directly in...and then through a pedal I have that has a driver transistor in the circuit even in the by-pass position. The results were fairly startling. The bass direct sounded it's normal (what I've been used to) kinda dull sounding. Through the pedal, it was very bright and crisp.
Most of the time I've played the K200 rig direct. I like the brighter sound though with the pedal in the loop, so I guess I'll be playing the K200 with the pedal in front of it till I figure out a way to raise the input impedance of the K200.
I did notice that even on this early K200, the "flattest" tone control settings are: Bass at 12:00, and Treble at about 9:30. So, if you set both tone controls at 12:00, you automatically get a considerable treble boost.
The transistors these days are SO much quieter than what they had available back in 1970...so the noise is much less of an issue today. I seem more hum than hiss now after replacing a lot of the transistors in this K200.
I may have to exparament with changing the input load resistor on one channel and see where that gets me.
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: Input impedance / load? K200 [message #21082 is a reply to message #21077] |
Fri, 08 August 2014 10:57 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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And BTW, I tested my repaired/refurbished K200 last night by actually playing music through it, and the results are AMAZING! It's once again a BASS BEAST!
Ya, I know, there are a lot of louder bass rigs out there (I also have a 500 watt Crest powered rig), but nothing has the "COOL FACTOR" of a Tuck & Roll Kustom!
Thanks a bunch to you and Chicago Bill for the guidance and tips.
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: Input impedance / load? K200 [message #21084 is a reply to message #21077] |
Sat, 09 August 2014 00:18 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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You are correct sir!
That buffer (which is in place even when the pedal is in bypass), allows the guitar pickup to see a very high impedance load (like 1meg ohm)...and feeds the amp with a fairly low source impedance...so it's unaffected by the relatively heavy 50k ohm load of at the input of the K200.
The gain of the pedal is unity in bypass mode BTW...but the level of the bass is affected considerably by the heavy load of the K200. This is all consistent with my findings when designing better quality DI's.
Give it a try. The difference is quite amazing. It's like turning the treble control up about 2 or 3 hours worth.
After more experimentation today, I've noticed that the effect of the heavy load varies quite a bit from instrument to instrument. My Ric 4001 is affected much differently than my P-basses.
This is clearly an area where the older Kustom heads could be dramatically improved on.
What does your Frank head have for input resistors Steve?
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: Input impedance / load? K200 [message #21086 is a reply to message #21077] |
Sat, 09 August 2014 08:42 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Yes, the SS buffer in that way is like a cathode follower in the tube world of audio, in some amps it sounds good in other amps I prefer the type of tone stack drivin off of the plate!
The Frank heads are a bit different in that off of the input jack is a 100k volume pot followed by the blocking cap and then a tapped stack of a 47k and 200 ohm resistor.
My main bass amp for 28 years has been a k250 head driving 2 EV 15" drivers, one L model and one B model.
A little over a year ago Had picked up a K300 pa head for parts and I ended up stuffing the K300s power supply and 150 watt output stage under the K 250s hood.
These PA heads use a choke in the power supply and allow the amp to sound far puncher than it's 150 watts rms would lead you to believe !
As you would think it is on the heavy side now weight wise and due to the the spare current on hand from only having to power 2 channels instead of PAs 6 channels and EQ /. Feedback reduction circuit the output stage pumps out over 165 watts of clean power.
The pickup impedance thing can make, like you said a big difference!
My single coils in my Gibson SB 350 bass and the singles in my Hagstrom bass sound a good amount different but the tone controls can more than make up for it, but both great! The big ass humbucker bridge pickup in my Carvin is very fat sounding, may be too fat, but the old set of active EMG Jazz pickups that I have in my 78 P bass really does it all for me weather I am playing Yes tunes with a pick, or Motown R&B with my fingers!
[Updated on: Sat, 09 August 2014 10:13] Report message to a moderator |
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| Re: Input impedance / load? K200 [message #21088 is a reply to message #21077] |
Sat, 09 August 2014 16:41 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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That's funny Steve! The band I played with back in the early 70s (and still play with today) had a K300 PA. We used it from 1970-1973. I have no idea what ever happened to it.
I never thought about a power supply transplant...that's a great idea! Are the voltages in the K200, K250, and K300 about the same? I guess a higher current power transformer with less sag would make for more watts pretty easily.
It sounds like the Frank head would have about a 100k load on the pickups, which would be better than the 50k most of the later K200's have. Your active EMG '78 P-bass wouldn't care though. Neither does my active Fender Lyte P-Bass. I really like that little bass BTW. Sounds good whatever I plug it into, and it's easy on the left shoulder. Plays real nice too.
The basses that seem most affected by the heavy load of the K200 are my '64 Precision, and my Ric 4001. But they are affected in different ways. The Fender only loses it's high end high crispness, and sounds dull...but still plenty of low-end. The Ric loses both it's high and low end when plugged directly into the K200. They both sound totally different when going through the pedal, even when it's in bypass.
I'd really like to get the K200 sounding better on its own. Not that it sounds bad...a K200 never sounds bad (especially since the rebuild), I just like the extra crispness when the instruments aren't loaded so heavily.
I was even thinking about hanging a single FET buffer right at the input jack.
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: Input impedance / load? K200 [message #21127 is a reply to message #21077] |
Mon, 11 August 2014 20:23 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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I actually built one of the FET input buffers today using an MPF102 jFET.
It works stunningly well on the bench. Yields a load on the instrument pickup of about 1.1 meg ohm...which should make my P-Bass much happier than 50k! Unity gain. It doesn't clip till about +15dbm, so I doubt even the hottest guitar pickup will cause clipping. Noise and distortion is quite low...about 0.03% passing zero dbm test tones.
It's built on a perfboard thats about 1 inch square, so it should easily fit right behind the input jack. I put a 3 pin, 15 volt regulator on the board too so I could power it from the +22 volt supply in my K200. Max voltage on the MPF102 is 25 volts, and I didn't wanna push it that close to the limit. Most of the data sheet tests on that FET are done at 15 volts, so I figured that was about where it wanted to live.
My plan is to put it ahead of one of the two input channels on my K200...leaving the other channel fully stock.
I'll report back after I play some music through it.
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: Input impedance / load? K200 [message #21138 is a reply to message #21077] |
Tue, 12 August 2014 11:07 |
daveobergoenner@gmail.com
Messages: 104
Registered: January 2007
Location: Missouri
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Senior Member |
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Last night I installed the FET input buffer in my K200, right behind one of the input jacks. My, what an improvement!
Much more high end when the pickups aren't loaded at 50k ohms, and the low end is much fuller too. Almost sounds like I installed a tweeter in my 2x15 Kustom enclosure with JBL E140'S. It's a keeper.
We also tried it with a Strat I have, and the results were similar...brighter and fuller.
Of course, if you use a pedal, or an active instrument, you already have a light load on your pickups. But if you play straight into the amp input with a passive instrument, you might wanna give this a try.
I used this schematic...
http://www.muzique.com/schem/eagle.htm
And just added a 15 volt 3 pin regulator.
Dave O.
Dave 0 Wall-O-Bass
Webpage - <a href="http://webpages.charter.net/daveobieone/" target="_blank">http://webpages.charter.net/daveobieone/</a >
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| Re: Input impedance / load? K200 [message #21139 is a reply to message #21077] |
Tue, 12 August 2014 11:36 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Hey cool results and good to know , but for me I must note one thing in regards to the bass JBL drivers you are judging thru, as have always found them a tad too dark for me for Bass no less for guitar!
When I have used them for Bass it was always with a another driver in the cab with better highs and or two cabs with one having drivers with better top.
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