Menu

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 - elwood

#1
Hey Dellamorte,

Good work copying all those nets from that old drawing. I was studying the original L5 circuit a while ago and found some of it very hard to read. (low-res scan I guess)

Your schematic looks good, but may I politely suggest a few things that will make your drawings even better? These are my personal opinion of course:

- Don't use angled lines for wires. It's better to make a bit more room and take a few right-angled turns. Lining up stuff visually really helps make things look neater.

- Beware of text that is obstructed by other parts. The text is important and I always like to make room for it. When you print out your drawing in black, text that is covered by wires or other parts will be unreadable.

- Further to the idea of reduced visual clutter, I would also say to use as many straight lines without turns as possible. Usually this just means nudging a few parts so that lines can connect straight into the part. See for example your TREBLE pot, or the MID-FREQA and B pots.

Before I start into too much checking on a complicated circuit I usually do the things I've listed above. I find that with less visual clutter and a more "readable" circuit drawing, errors are much easier to spot.


Cheers!
#2
Hey Teemuk,

Thanks for the detailed response. I certainly realized the "not enough is not useful" side of things. Since many of configurations didn't yield much difference in sound at all. But I wasn't sure how far most amps go with it. Most of my design projects for work and play are industrial/utilitarian stuff and a lot of things with micro-controllers. I'm really finding the need to listen and decide what I prefer to be a gratifying and also frustrating part of the design process.  :)

I think I might have (from some trial and error) come up with values that I like. I just have to validate that they're reasonable.

Just out of curiosity, what software do you use to simulate circuits? How about for schematic and PCB layout? I've been learning Proteus at work, but it's pretty expensive for hobby use.
#3
Hey everyone, I've been working on an 1x12" amp design and learning a LOT in the process. My "reference" amp is a Fender Hotrod Deluxe which I do like. I'm not trying to emulate it with solid state circuits, but it has been a good reference point so far.

One thing that's baffling me is the "presence" control. On the Hotrod I always have it cranked... I find things sound terrible otherwise. I've plotted the curve of the Fender power amp with the presence control cranked and have been trying to get something similar in my power amp. I know that the presence is part of the current feedback circuit, reducing the feedback at the higher frequencies. I'm having trouble though when it comes to knowing how much feedback I should be using in my own circuit.

With respect to current-mode feedback, I have implemented it around my amp using a 0.25R resistance in series with the 8ohm speaker. I have buffers and such set up to amplify the recovered signal from the shunt and mix it back into the signal feeding my amp. I use a dummy load that is switchable to 8, 12 or 24 ohms and I've been using this to see how the load affects the output signal from the amp. When I change from 8 to 24 ohms the amp output changes by about 3-4dB based on my configuration. But I have no idea what is the "normal" amount of feedback to apply, and how much my load should affect the output voltage of the amplifier.

Can anyone explain what is the normal amount of feedback and how much it should affect the output of the circuit? So far I haven't come across any hard and fast numbers based on practical implementations. Thanks!
#4
Hey all,

I'm pretty experienced with the classic filter types and putting together various active and passive filters for simple things. But here's something I've recently run into that I haven't experienced before: converting a known (desired) complex EQ curve into the simplest circuit possible.

Following the advice of something I read, I've been using a 31 band graphic EQ to shape the sound going into the power amp section of my project. I've also found a great windows program called Tombstone for measuring and printing response curve of real circuits. But I'm having trouble figuring out a systematic (automated, programmatic?) way of producing a circuit that has roughly the response of my desired curve. Do I make a gyrator for every peak and valley and tweak each one like in a graphic EQ? Or should I make multiple stages, one for each boost/cut? Simple rolloffs are easy, but I need a few bumps and dips in the response here and there.

What method to people use for this sort of thing? Any insight would be greatly appreciated.
#5
Amplifier Discussion / Re: Grounding techniques
February 29, 2008, 08:01:25 PM
That's a great tutorial... thanks for posting it.

A note about PCBs and ground layout. I've done some high frequency video PCBs (>100MHz) where grounding, inductance, power supply bypassing is a real challenge. This, and designing for EMC compliance (which doesn't apply to analog audio designs.. at least in North America) has found me in the library and reading a lot of app notes about layout. What I've realized is that there is a lot of misunderstanding, but if you are more careful than you think you need to be, the results will work.

Bypass capacitors do a lot to stabilize power supplies near both digital and analog ICs. This can help a lot of misbehaving circuits. It's not a replacement for good grounding, but while we're on the subject I think that good power supply wiring and good grounding go hand in hand. Keep in mind that filter and bypass capacitors don't work at all frequencies which is why you often see several types and values side by side. Also remember that just like grounding, power supply sags in your circuit can cause unwanted oscillations in analog circuits and troubles in digital circuits too.

I personally like ground planes very much. With even just a 2 layer PCB dedicating most of the bottom layer to grounding can improve the success of layout because you're always just a via away from getting at ground. A common method of layout for boards with high and low power sections, or digital and analog sections is to partition the ground planes and connect them together at a single point. You need to be careful when running traces across the gap, but this lets you have the advantages of a ground plane for each section, and the advantages of the star or bus grounding scheme for the entire circuit.

Anyway, there are no golden rules in this game, but I'm just sharing with you my experiences. I suggest that everyone take apart stuff that works and have a look at how the designers did things. You might not like everything you see, but you'll learn a LOT about how much care in grounding and signal routing is enough for a specific type of circuit. You might be surprised what some designers get away with. There's nothing wrong with over engineering something as long as it doesn't significantly impact the overall cost. But there certainly is something wrong with a circuit that doesn't behave itself!

I know it's pricey, but I highly recommend the book "Noise Reduction Techniques in Electronic Systems":

http://www.amazon.com/gp/product/0471850683

It's old and not a very big book, but don't be fooled! There's a ton of great information in it. I found it quite an eye opener! There was a lot of stuff in there that I've never come across on the web.
#6
Amplifier Discussion / Re: Class D Amp Recommendations?
February 27, 2008, 09:51:21 AM
Those Tripath ICs certainly have very good specs. Note that according to Wikipedia: "Rather than being a separate "class" of amplifier, Class T is a registered trademark for Tripath's amplifier technologies. It is an implementation of Class D amplifiers..." So essentially it's just a better implementation than other companies are currently doing. Looks quite fine, but another part of that article suggested that Tripath filed for bankruptcy protection last year... seems like a bit of a risk to use one of their parts in a new product.

What were the problems that you had with Class D?
#7
Amplifier Discussion / Class D Amp Recommendations?
February 26, 2008, 07:51:16 PM
I'm trying to keep my SS amp design super efficient (I want it to run on batteries and be super light) so I've been checking out Class D amp ICs. I know they've gotten a bad reputation in the past for being high distortion, but it appears that new ones are getting pretty good. Obviously you want to keep it out of clipping and the PCB layout and output filtering is a bit more of a challenge, but the power supply and heat savings are very impressive.

So, I've been checking out the TPA3106D1 from TI. Looks like a great part, and works with a single supply. Can do >30W into 8 ohms with <1% THD from a single +24V supply at >90% efficiency.

How much power do most people feel they need for a solid state power amp? Mine will be driving a 1x12" open back combo with a speaker rated at >100dB SPL 1W/1m.

Does anyone have other recommendations of parts or have experimented with Class D at all?
#8
Amplifier Discussion / Re: Boutique Solid State Amp?
February 25, 2008, 05:48:13 PM
Hey teemuk,

Thanks for your comments. BTW, I've been looking through your amplifier book and it's fantastic! A lot of hard work putting that together I'm sure, but it certainly does have a lot of great stuff. Judging by your understanding of audio circuits I wouldn't expect you to buy my amp. :)

Maybe it's a foolish dream to want to make a living building musical equipment, but I figure I'll start by making the kind of amp that I wish I could buy, and then see who else is interested. My approach in designing technology (which I do in non-musical areas as a job) has always tended to focus on the end user, features (or lack thereof), and how it solves problems or offers capabilities that haven't been possible before or weren't easy. Which is why I'm sorry that so many guitarists choose to prefer one type of technology over another instead of looking at what a piece of equipment will *do* for them as a tool for making better/new sounds.

Marketing.... now that's something I have to work on....
#9
Amplifier Discussion / Boutique Solid State Amp?
February 25, 2008, 01:16:06 PM
I'm very happy to have stumbled upon this forum. Having grown up in the 80s and learned about electronics long after tubes were in museums, it's great to find others who are interested in solid state amps! I've been working in the electronics business for a few years now mostly doing designs of non-music stuff. Due to sheer boredom and the desire to make musical instruments I've started working on some designs for guitar electronics. My main project is a highly portable 1x12" combo amp with a number of interesting features. I hope to make this into a "boutique" product as a side business to support my hobby.

Strangely enough as much as I love modern electronics, I got roped into the tube craze when I bought my current guitar amplifier. (a Fender Hotrod Deluxe) It really isn't all the great as a practical amplifier. It's too loud for practising, (which I've mostly solved with an attenuator in the effect loop... the speaker attenuator I built doesn't seem to sound as good) doesn't have a headphone jack, has a spring reverb (which I hate the sound of) and generally has a very small range of usable tones. (not good for an eclectic musician) Also, I can't ignore the fact that it weighs 45 lbs and is terribly inefficient.

So when starting to design my amp I pretty much ruled out a tube power amp due to weight and size. And then that left the choice of a tube vs. solid state preamp. I built a little tube test system for quickly (and safely) setting up circuits with a pair of 12AX7s. My ear tells me after a lot of experiments that I don't necessarily like the tube sound better. I've got some fairly good sounding preamp circuits working with opamps, and a nice small hardwood ply open-back box with an Eminence Lil' Texas (neodymium magnet) speaker inside.

My question for you all is this:

Can a solid state amp work as a boutique, high-end product? Would you pay $1000-1500 for a low-run, great solid state combo amp that has exceptional sound and features?

Since I've designed a lot of commercial stuff, I can't think of a hobby project as a one-off anymore. Instead I want to build stuff that other people will want to purchase and enjoy. It all seems more worthwhile that way.

All comments are greatly appreciated!